Sometimes however people get put off with dog training before they start.....
Took my young Doberman to training class. Behaved very well, then a smaller dog decided to bark at him and try to attack him. Bless him, the Dobie just barked back and tried to ignore the fangs at his throat!
The outcome? Trainer shouted, (to us) has your dog always been vicious ?? (because he barked!)
We turned round, and walked out! Never went back.
So please, if you are going to be a dog trainer, a plea - don't be prejudiced against the big, or black and tan breeds, look at the behaviour and who is being aggressive and who isn't when there's any incident!