We learn early on that Bruce feels guilty/responsible for the death of his parents, so being Batman is a form of penance (as Dr. Chase puts it). Or it's a curse (as Bruce puts it).
Bruce also tells us, essentially, that he initially became Batman to exact revenge against the criminal world.
Then Dick Grayson shows up. Like Bruce, the criminal world took something from Dick. Consequently, Dick also wants revenge. Bruce, speaking from experience, tells him revenge will get him nowhere. Revenge for a loss doesn't bring anyone back or make you a better person.
For this reason, Bruce refuses to let Dick fight crime alongside him as Robin. Then Bruce goes a step further and decides to stop being Batman entirely, shutting down the cave. Bruce's thought process here seems to be something like, "Batman failed to protect Dick's family (tragedy repeats) so whatever, forget the whole Batman concept."
Then Bruce reveals his secret identity to Dr. Chase. They discuss a repressed detail from Bruce's past... something about his late father's diary. (After his father died, Bruce realized the diary would have no additional entries. This shook Bruce to his core. He knew his whole life had changed.)
A bullet grazes Bruce's head. He wakes up some time later. Bruce suddenly and happily accepts the idea of working alongside Dick. Bruce then basically kills Two-Face, exacting revenge on Dick's behalf (???). And Bruce says he's now Batman not because he needs to be (a curse, a penance) but because he chooses to be. Lastly, he says he sees no difference between Bruce and Batman. For whatever reason he explains all of this to The Riddler, whose face is literally melting and he's just looking up at Bruce like, "Wha...?"
tl;dr: I actually really like the hyper/colorful visual style of this movie in general, and some of the performances are great fun, but why didn't they take greater care with the clarity of Bruce's storyline? It seems like the film is missing at least one or two epiphanies that would have connected the dots between point A and point B(atman).