How else are you supposed to meet people?
School. Through friends. Clubs, social events, bars, out in random public places.
Fact is, most relationships fail. Many people do not take this failure well. Many people whose relationships fail (especially young people) tend to want to make the other person miserable. Work is the perfect place for this to happen. There are other kinds of stresses working with your significant other can have that have an adverse effect on the relationship.
Now, it's one thing to sleep with someone you work with every so often, or to go out with them once or twice. But to engage in an actual relationship with someone you work with, especially with a supervisor or subordinate isn't always the smartest thing in the world. You'd better be damn sure the person you're with isn't a psycho hose beast. I'm not saying it always ends up badly, but most of the time...it's not the best idea in the world.