This question is one of my favorites. I have gotten this question from people with cute new puppies, dogs they have had for years, and rescue dogs, basically everyone will ask this questions. The answer is it depends. Wait... did a dog trainer really just say you don't always need a dog trainer? oh yes I did. A lot of it depends on what you want from the dog and, in all honesty, a lot of it depends on your budget. 
If you don't care what your dog does and if it listens to you then no need for any training. If you care but can't afford a trainer then you learn as you go, and aren't we all glad for youtube and google. However if you can afford a trainer and you want a well behaved dog I recommend hiring one. 
But wait a moment, even if you have trained your own dog before you should hire a trainer? I recommend it. why? Because not all dogs are the same. Individual adjustments to training are a good idea. The trainer also provides a second pair of eyes, that shouldn't be biased, that can help you catch things you've missed. This is why I don't sell packages of 10 lessons, but rather set things up with you on your dog's needs. There may be some dogs I only need to work with once or twice- basically doing some finishing touches. There may be some dogs that I need to meet with 10 times and help you get it under control, and of course there is the happy middle ground. 
To go with this, with my dog Stinker, the beautiful malamute on the banner, I have had several of my trainer peers work with her with me. What? A professional dog trainer enlisting other professional dog trainers? yes. They provided me with some insights that I was missing, because literally, I was too close to the situation. 
However again I want to note, in some cases, if you can't afford a dog trainer and/or you really want to do it yourself with the aid of online resources your dog will most likely turn out just fine. But I will always recommend considering getting a dog trainer at least to give you some insights you might be missing. 
Now if your dog is having aggression issues you need a trainer. if you dog will not respond to your commands you need a trainer. If you want to do a specialized certification you need a trainer (they have to sign off on it). If you dog is having other behavioral issues, for example keeps having accidents in the house, you need a trainer. I am being blunt. IF there is some issue you cannot get resolved, don't give away the dog, get a trainer. 
One of the most ridiculous stories I have ever heard was when a family went camping with their 12 month old lab. They went to go on several hour activity and left the lab in the trailer unkenneled. Now this is also the first time the dog had ever been in the trailer. Anyone want to guess what happened? Yep you got it. The trailer was destroyed. They decided the problem was their dog and they needed to get rid of the dog. No let me explain to you what I had to fight myself not to explain to them. They were idiots. A young lab is very chewy. Especially when they are nervous or stressed. Guess what leaving the dog unattended for hours in new environment=mega stress. Now if they had had the dog kennel trained and in its kennel there would have been no problem. Now did the mean to be idiots.No. They honestly didn't know it was their first and according to the parents only dog. They found the dog a new home. I applaud. and that is the end of their dog story. 
Now in my opinion what should have happened is they should have 1. been more observant and conscious of their dog. 2. IF they had been working with a trainer. any trainer worth their salt would have had them kennel train their puppy and would have walked them through appropriate times to kennel your dog. That camping experience would have been an appropriate time to kennel their dog. Even after the destruction tornado ransacked their camper rather than dog's bad they should have realized they screwed up and worked to train the dog so it was never a problem again. That is what the responsible dog owner does. They figure out what they need to do better and train the dog so there is no longer a problem. They do not get rid of a dog.  
Anways, now that rant is over. Lets do a quick recap. In some cases you don't have to have a trainer, but there is not a case I am aware of where having a trainer is going to hurt. Also in most cases it is extremely beneficial. 
Hello world,

One of the most important facts we can learn about a person is what motivates them. This motivations gives way to what their end game is. You start piecing that together and real quick you can tell if its someone you want to have around, trust, hire, etc. Most cases I've seen companies, sometimes people on an individual level, create these lovely Mission Statements. I feel that is a bit too flowery for my mode of operation however, let me explain what drives me, and we'll consider it an even trade. 

Step one: I love animals always have. I always wanted/ still do want any and all animals I can get a way with. My mother lovingly calls my apartment the menagerie and my husband is often trying to find ways of dissuading from getting another pet. (please note all of my animals are loved and well cared for. They include My beta fish Washington, my bearded dragon Bell, and my malamute Stinker.) 

Step two: Growing up I always wanted a dog. Technically we had two, Indiana Bones and Omni. Both were very short lived for various reasons, Indiana Bones ran away and Omni we gave to a nice lady because my parents felt we just didn't have time. However those were not the only two dogs in my life. I grew up in Central Illinois (NOT CHICAGO). It is common practice to dump animals out in the country that people decided they did not want. There were many dogs that I found, some had gone feral some had not, that I took in and worked with until animal control could come and pick them up. What broke my heart was not having to turn them over to animal control, although that hurt plenty too, but rather it was they were out there in the first place. 
In North Central Utah we don't have as much a problem with that as we do with what I call "serial humane-ist" these are people who get a dog, take care of it, and then when something goes wrong goes and drops it off at the humane society, pound, etc. On the outset this seems less mean than dumping it out in the country, but I argue it is not. 
Both issues end with animals dying needlessly, and for those who survive being emotionally scarred to some level. Animal control is not the solution it is a band-aid. The solution comes from owners/pet parents, whichever term you prefer, taking responsibility. 

Step Three: I have found that people don't take responsibility for three reasons: 1. they honestly don't know how/ any better 2. they can't because of situations outside of their control 3. they don't give a damn. Reasons 1 and 3 are by far the most common. But lets do a quick break down. Category one includes mostly first time dog owners. They are people who mean well, but just don't know dogs. Often I have found they will get upset and worried over minor things and miss when there is actually a BIG problem or worse yet that they are setting the dog up for a BIG problem. However in the same category are people who think they know dogs. Example they think the best way to punish Fido is by hitting him on the nose with a newspaper. What they don't realize is they are causing crazy aggression issues to build up with Fido. They are setting Fido up to bite them, someone else or something else. They have effectively created a ticking time bomb. Congrats. Now people who do that are not all evil. Some people honestly do it because that is what grandpa and dad did with the dogs growing up (I'm not sexist you could include grandma and mom on that). Now this is where I come in with group one. A lot of research has been done in the last 20 years, primarily by the American College of Veterinary Behaviorists (Please visit their site Their latest book "Decoding your Dog" is in my mind is the holy text for dog training. Okay back on point, I utilize my resources to train the dog and educate the pet parent (my favorite term) so that the dogs and owners can have a positive relationship and the dog stays with its family.
Now real quick before I address the second group. When you get a dog you should be making a commitment to that animal for the entirety of that animal's life. If you are not able to do that you should not be getting a pet, especially a dog. 
Group two are rare. I mean really rare. A lot of serial humaneist think they are group two. They are not. Things out of your control include serious illness, for example severe postpartum, cancer, etc; children with severe behavioral issues that put the dog's life and well being in jeopardy, and moving out of the country. There may be a few other odds and ends but these are the big three. May I state moving apartments is not a valid reason to get rid of a dog. If you have a dog and you are looking for a place to live you need to look for places where you can have your dog. Do not look at options that would exclude your animal. IF you do need to get rid of your animal take responsibility. Find it a home yourself. Don't dump it in the system. It is more likely to live a happy life and be less traumatized this way. Please note that as for training assistance for the big three there are situations where special training can be given that can help turn the dog from a hindrance to a help. However, those go by case by case basis. 
Group three are people who don't give a damn. These range from people who are gifted a dog to people who abuse animals. For the happy end of the spectrum (people who are given dogs not of their choosing) Training can help them bond and learn to work with the dog. I have worked with several in this situation and by the end of training they would never even consider getting rid of their dog. For the other end of the spectrum, the majority just need to never be allowed dogs. Some could make changes with education... but for the most part they won't . 
Now a lot of my breakdown has had to do with people who get rid of their dogs, but this also goes for people who don't get lassie fixed and lassie gets pregnant, don't take care of aging dog, etc. Be warned I will use a lot of what I am saying in this as bases for other posts. With that in mind lets do a quick wrap up.

The motivation to be simple is this I have seen problems with animals/people relations I have seen both suffer because of it. I have also been able to work to help fix those problems and seen both a lot happier in the end. That is why I do dog training. I do it so people and their dogs can be happy together. End of story.