Although we become well acquainted with Travis throughout Taxi Driver, his mental instability makes his actions unpredictable, and although Travis seems sympathetic, we never fully understand him. Travis is never part of the normal world. Though he initially wants to fit in and to be like other people, he is too mentally ill to act normally. Even at his best, at the beginning of the film, he can't sleep, drinks heavily, pops pills, and spends his mornings in porn theaters. After Betsy rejects him, Travis becomes hysterical, violent, and obsessive, and from here descends into madness. He loses all self-awareness and deludes himself into believing that shooting a presidential candidate and then shooting himself is a heroic gesture. Travis changes from a wounded man into a hardened one, testing our sympathies and distancing himself through violence. Young Iris prevents Travis from turning into a monster by giving him a reason to look at the world outside himself. Even as Travis plots his heroic act of violence, he worries about how to save Iris. He believes he has cut himself off from all worldly feelings and that he is just training to be a soldier, but his concern for Iris suggests otherwise. Travis's many contradictions make him one of the great characters in film history.
We never learn exactly what happened to Travis during Vietnam, and the rest of his past remains unexplored, so there's no way to explain why Travis has become the way he is. His war experiences must have influenced his character, acquainting him with violence and helping to turn him into a killer. Travis's anger wouldn't be so frightening if he wasn't able to transform himself into a warrior so efficiently. When Travis goes to kill Palantine, he sports a new Mohawk haircut. The 101st Airborne paratroopers made this a popular haircut for American soldiers to wear into combat when they flew in on D-Day in World War II, and Travis's Mohawk shows the influence of his experience in the army on his character. Travis has also been influenced by his parents and his upbringing, though we never catch any glimpses of this past. His obsession with and disgust for all things sexual are surely rooted in early experiences, and his many comments about destiny or being chosen by God suggest that he may have had a religious upbringing as Scorsese and Schrader did. We know little about Travis outside of his taxi, and he remains a mystery.