Quite often, being a rabid movie fan, and devouring most movies that come out locally, because only 4 or 5 are out at a time in the nearest cinema and they normally run for a while, and of course there are a lot I don't want to see, so my friends could be at those while the movie I was watching was on. Anyway, another reason is because when I want to see a movie, I want to see a movie, and the tastes of my friends aren't going to stop me from seeing it.
One example is Inception. All my friends thought it made no sense whatsoever, so I'm going to see it pretty much ASAP on my own.