When I first got V I was 19 years old and he was a pup. A year or two later he was full size and for lack of a better way to say it, out of control. At the time I didn't want to do training because I liked him for who he was and didn't want to change his personality.
Fast-forward to today and a lot of people feel the same way I did about 10 years ago.
You can probably imagine that we (adgtk) are in favor of doing training. This is because we are able to help people with their dog's unwanted behaviors without having to change their dog's personality.
One of the reasons for this is because we use a lot of rewards when training. These rewards provide motivation and also make training fun. The other reason is because we are not using anything that hurts or scares dogs. The side effect of pain and fear in training is that you can change your dog's personality and that is where this thought process comes from.
How does using pain or fear change a dog's personality?
For starters, your dog will figure out that trying any behavior (besides the desired behavior) can result in something painful or scary. (e.g. a shock, jerk, being yelled at.) This often results in a dog that is not willing to do any behavior for fear of the consequence. This can take a happy-go-lucky dog and turn him into a dog that is reserved and even nervous.
It can also change a dog's personality because the dog can start to develop negative associations with what he thinks is causing the "correction." For example, if a guest enters your home and your dog jumps, and then is immediately "corrected," your dog can associate the guest with the correction. Over time, and even for some dogs more immediately, a negative association is developed and that association leads to fear, which can then lead to aggression.
To summarize all of this, training can be fun for all and will not change your dog's personality if it is done correctly.
Kevin Duggan, CPDT-KA