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: This is just my opinion. Remember Iris's line "Have you ever tried looking at your own eyeballs in the mirror?" or something to that effect. Well, Travis sees the madness reflected in his own eyes, doesn't like what he sees (as it reminds him of what he has done, what he might yet do).