Love, Biologically

 The biological purpose of love is to transfer resources to the loved one. Reciprocal love makes no sense; don't play Pong with the resources, that's plain dumb. Hence, men love women, women love children, and children can barely manage to love themselves. 

 Men aren't really supposed to love children. They're supposed to love sex, and then, as an inevitable consequence that they don't think about too much, they end up with kids. Then Darwinian instincts force them to provision the children, even though they would really prefer not to. Women don't love kids very much either; they're supposed to love themselves, especially when they're young, because they can just have another kid. The idea is to do the bare minimum so the kid doesn't actually starve to death or try to make friends with a crocodile. If it doesn't work, eh, oh well. It's not like her man will stop impregnating her, so the brat was going to be replaced anyway...

 Both men and women are largely forced to become attached only when it seems they will develop a reputation for disloyalty if they don't. This process is extremely buggy, since human perceptions are so unreliably even other humans can't figure out what insanity is going through their head. The loyalty-mimicking genes have to fire strongly since the disloyalty sensors are sensitive, which results in rather chaotic and unpredictable attachments.