Why do people girls go after the "bad boy" who treat them badly, but they don't seem to see it?

In other words, why do women (really any gender combo could work for this question) fall for the "bad boys" who treat them horribly, even after friends and family point the bad lovers actions out. Some people tend to make lame excuses for them and ignore the inexcusable wrongs. They never seem to be able to to answer the question "Why do you want to stay with them?". It doesn't make much sense to me. Keep in mind that I'm talking more about teen aged girls then any other demographic. One thing I don't get in the case im witnessing is that the boy the girl likes is not even popular which is the most common answer I get to this question. That answer being "Girls like popular guys".