In most media, Bruce Wayne becomes Batman because he wants to to strike fear into criminals because he believes it has a greater impact than just pummeling them into submission.
That is why he dons the cape and cowl. That is why Bruce Wayne became Batman, the demon who strikes from the shadows and to make sure no one goes through what he went with his parents, and to ensure no one else dies.
When it comes to the Nolan Batman trilogy, he has the same reasons and motives on why he becomes Batman but in Batman Begins it's early in his career, which was cool, then in The Dark Knight he has been Batman for 2 years. Then in the movie he keeps considering retiring to be with Rachel and leaving it to Harvey Dent.
WAYNE The day you once told me about, the day when Gotham no longer needs Batman. It's coming.
Rachel looks at Wayne. Conflicted. He moves closer.
RACHEL You can't ask me to wait for that.
Wayne takes Rachel's arms, looking at her, excited.
WAYNE It's happening now- Harvey is that hero. He locked up half the city's criminals, and he did it without wearing a mask. Gotham needs a hero with a face.
Then in The Dark Knight Rises he quit being Batman for 8 years and gave it to the cop John Blake and says Batman can be anybody?
WAYNE To protect the people closest to me. BLAKE You're a loner with no family. WAYNE There are always people you care about. You just don't realize how much until they're gone. (Collects himself.) The idea was to be a symbol. Batman could be anybody, that was the point.
In most Batman media, he protects Gotham as Batman for many years until he gets old, like in the cartoon Batman Beyond. He also swears an oath to protect his city at all cost like in the _Batman Telltale Series\ where Bruce Wayne explains to Tiffany why he does what he does, why he can't ever afford to take a night off.
Tiffiany: Oh how can you go out there again after what just happened I don't know how you do it. You saw those people get massacred and that's just the start
Batman: I do it because I can't afford to take a night off. I do it because people rely on me to be there when things like this happen I do it because I'm Batman.
Also, in Batman Forever, Val Kilmer's Batman explains why he chose a bat:
BRUCE: And there, deep in the shadows......I saw...It was coming toward me. I was scared at first, but only at first. The figure in the dark was my destiny. It would change my life forever. I would use its image to strike terror into the hearts of those who did evil. I would ensure what happened to me would never happen to anyone else again. I would have my revenge.
The looming threat on Gotham reaches its boiling point, forcing Bruce to decide how far he is willing to go to protect his city from a force that threatens to bring it to its knees.
So let me ask this:
Why is Bruce Batman for only 2 years/six months, retiring in The Dark Knight Rises for 8 years.
And why does he consider retirement throughout The Dark Knight?