Thanks to The Matrix. Before that I don't think I really did question what was real. But after watching The Matrix I have started to seek out theories and different thoughts about reality, especially in TV shows and films.
I love Serial Experiments Lain (it's an anime if you didn't know) because it deals with reality and whether Lain, herself, is real. Higurashi no Naku Koro ni is pretty much the same sort of thing, but with violence ^_~. Paprika (another anime but this time a film) also deals in reality stuff in a pretty interesting way.
But I think I question what is real because I have so many unanswered questions. Like where did we come from? Where did anything come from? If God exists then where did he come from?! It's maddening! O_O