From what I heard, the person who established Athenian democracy did so after first overthrowing the previous ruler in a civil war, having concluded that becoming powerful was the best way to become a Great Man. He then reasoned that, since everyone should strive to be a Great Man, then everyone else would also be obliged to do the same thing he just did - which would mean endless civil wars. Which would be bad. So he came up with the clever solution of making everyone a ruler, so they could all be Great Men without having to kill each other first. Hence, democracy.

Or something like that, anyway. Wikipedia doesn't say all that much, so I suspect that the story I remember is more story than actual history.