I think it's part of his mommy issues - he's attracted to women whom he has to care for in some way, but who are, in other ways, stronger than he is. His caretaking streak can be one of his best and noblest traits, but it can also be really self-destructive and lead to awful co-dependent crap. That's one of the reasons I find the possibility of Spike/Dana kind of interesting - crazy AND a Slayer; she's his dream girl *g*

Would go on at greater length, but I'm falling asleep.)