This one's not exactly a puzzle: Taxi Driver is about a taxi driver. Simple. There's more to say than just that, though…

As a taxi driver, Travis gets to see a lot of New York—and he doesn't like what he sees. He also lives an isolated existence, thanks, in part, to his job. Even though he can talk with his passengers, his connections are all fleeting. He doesn't have any stable relationships in his life: Travis' parents are apparently back in the Midwest, and based on the anniversary card he sends them, it doesn't seem like they've been in touch in a very pronounced way.

He becomes "God's lonely man" then, drifting around, longing for connection when he's not raging against the world around him. His occupation helps condition his character and determines the way he relates to the world (even though Travis' brand of madness isn't shared by most taxi drivers, obviously).