From the ever interesting Jane Galt about child rearing and careers

I’m not sure. If childrearing is a) necessary and b) as tedious as everyone assures me, then it strikes me that whatever feminine thrill women get out of doing it probably increases the happiness associated with the activity. And, based only on my own previous relationship experience, I’d imagine that socialization which reduces the number of areas that have to be negotiated probably, on net, makes marriages happier.

That would go a long way to explaining why opposites attract, if in fact they do.