@Mame,
Statistical and economic theory.
Lets say there are two evenly-divided groups. Nice people (guys and girls), and jerks (guys and girls).
Let's randomly pair 'em off as couples. And let's assume that gender is not an issue (hetero folks always get paired with the opposite sex, and folks who like the same flavor always get paired with the same sex.)
You end up with:
25% of couples are nice/nice
50% of couples are nice/jerk
25% of couples are jerk/jerk
Assuming jerk/jerk couples have the highest rate of breakups, nice/jerks a moderate rate of breakups, and nice/nice couples the lowest rate of breakups, you see that the pool of nice people rapidly shrinks, and the pool of jerks remains large.
Yes, it's not empirically validated. Yes, many people mask how much of a jerk they really are.