When has Bruce Wayne ever really been truly happy?
He banters with Alfred, Dick, Tim, Damian, various heroes. He has friends and allies, he has positive encounters as well as negative ones with various personalities.
Being Batman is what makes him happy. He's not happy all the time (and who is?), but by the time he's a fully formed character, that's what he wants to do with his life, and he has made this decision, and is content with it.
Just because you brood doesn't mean you're not ever happy. This is the issue most writers have. They paint being Batman as this massive curse, and he doesn't neccessarily see it that way.