From my own experience, and from what I've seen, there are many causes, as someone else said. I think the biggest causes are hormones and genetics. I think I have both problems, plus stress and lack of sleep. Some derms say stress or lack of sleep causing acne is a "myth." (I'm still not actually convinced that food does it. I'll eat healthy and like crap and still have pimples, either way.)
Thanks, eggheaRAB. It's not a myth that, after a night of 4 hours of sleep, my face is an oil slick, and in a day or two I'll have some big, fat pimples on my cheeks, near my nose.
Also, if you do some reading, you'll see the most common explanation: dead skins cells don't shed properly; the skin cells mix with oil in your pores, clogging them, and then the acne bacteria feed on this goop, blah blah, and you get a pimple.
That might be true for surface acne, but my acne seems to originate much deeper in the skin.