The theme of loneliness is pretty obvious. He looks from his taxi cab at a couple holding each other, he watches pornography in small, dark rooms, he watches the dancing couples on the teevee, he watches "instructional" movies on couples, he watches Betsy and her co-worker interact through a window, he watches "Sport" and "Easy" (although the scene in the room is one of the few not from Travis's point of view). Every step of the way, he is surrounded by human affect (however perverse) and he is
outside looking in. He wants it, he craves it, he thinks about it constantly in his isolation, and, finally, he breaks down... and reacts violently.
You guys should check out Michael Haneke's
La Pianiste 
It's hard to reallly communicate the recepient's response in text though. Something like "OUUUU!!!" 
