Trivia: Director Martin Scorsese makes more then just a cameo in this film, he's the passenger that sits with Travis talking about how he's going to kill his wife for cheating on him with a black man. He's credited as "Man Watching Silhouette".
Trivia: Robert De Niro worked as a cab driver for a month to prepare for the role.
Trivia: The assassination scene in this movie is said to have inspired John Hinckley Jr. to attempt to kill President Reagan. He did it to get Jodie Foster's attention.
Trivia: 12 year old Jodie Foster had help in her role from a real prostitute. The real hooker had a small cameo during the scene where she helped Jodie after Travis Bickle almost hit her with the taxi.
Trivia: For the more explicit scenes, Jodie Foster was doubled by her sister, Connie.
Answer: It helps him create a tolerance for pain. As we see, he certainly gets a lot of that dished out to him.