I don't want to tip the dog trainer.
That's not good. Why don't you want to tip the dog trainer?
Why should I tip him? He charges me too much already.
That seems fair. I mean if you are expected to tip the trainer, though, it is rude not to do it.
I'm not tipping him because I've already paid him and he doesn't deserve more.
Even if you paid for what he has done, people in those kinds of jobs do expect to be tipped. Not tipping them could cause you to not be able to use those services anymore. And then you will have to find somewhere else to take your dog.