Where did stereotypes come from?

The theme in one of my communications classes this week is orientalism.  I had to watch an interview of the orientalism guru and scholar Edward Said, which was absolutely fascinating to watch.  In having this topic already fresh in my brain, I began to think about stereotypes and how desensitized I am to them in my everyday life.  It's like every time I pass a group of people that would ordinarily be stereotyped, I imagine what someone would say to them. It sounds really weird, but I can't help but think of the stereotypes and the reasoning behind them.  Not limiting myself to any group of people, of course, I think about stereotypes for all groups.  I was in the gym and saw a student of Asian decent (or who looked of Asian decent) throwing a ping pong ball up against a wall and catching it.  It made me think of one of the stereotypes of people of Asian decent where they talk about Asian people playing table tennis all the time.   I don't really have a point to this example, but just to show a thought process.  I just don't understand how the stereotype came about.  What if I was throwing a ping pong ball, would someone think the same thing?  Probably not.  Maybe the student just found the ping pong ball on the ground and decided to throw it.  Yet, I didn't stop in my tracks when I thought of it/saw it and it helped me realize how desensitized I am to stereotyping.  It's sad.  I don't even have an ending to this one.  It's just something to think about I guess. Why are stereotypes even alive?  Do they come from any truth?  Are they just made up out of some random negative thought?