For me, it's how good she is in bed.
It's fairly easy to seduce a chick and get her in bed with a few dates, so when I find myself single I just go on a rampage finding chicks and trying to get them in bed - and once I find a girl who blows my mind (even if she isn't the most attractive), then I stick with her.
Of course, she can't be ugly. But if she's at least somewhat attractive and REALLY good in bed, then I'm in love and I get lazy and stop chasing skirts.
Until I fuck up the relationship and have to start all over again. ;)
Life's too short to have bad sex.
