Because of Zack and Miri make a porno. You know, you want to fuck your friend. The guy you met freshman year of college and still wanna fuck senior year. He might be going to dental school and you're going to med school but he's still super fucking hot and... Well shit.
Me: "Damn it, dude. I've thought you were sexy since I saw you at orientation."
Him: "Fuck, I've always thought the same about you. We should be friend fucking already."