Why is there a constant war between Marvel fans and DC fans?

The title says it all: Why is there a constant war between Marvel and DC?
I just don't understand it. Many DC fans tear apart Marvel movies before they have even come out
and the same goes for Marvel fans. Both sides use ridiculous reasons to trash movies like The Avengers or The Dark Knight.
Why don't people base their opinion on the movie instead of on the fact if it's a Marvel or a DC movie?

Edit: I'm neither a Marvel or a DC fan, I'm just a fan of great movies. Both Marvel and DC have made amazing movies, like The Dark Knight trilogy, Captain America: The Winter Soldier & The Avengers.