Seriously it irks me to the core, why do nice girls date jerks, losers and deadbeats. Especially if the girl already has so much going for her, an education, a really good job and a great personality?
Maybe because nice girls find it difficult to turn down guys, who are actually losers. Some girls are too nice that they can't be assertive and end up staying in the relationship with the hope of maybe changing the guy or seeing something good in them, like funnychick said.
Because it makes a person feel special to date someone that is an ass to everyone else but good to them. Problem is that the niceness wears off eventually but by then people are too deep and attached to the jerk.