This thing called Love;
"A baby zebra can scramble to his feet and walk soon after birth, and indeed most other animal babies hit the ground running. But human babies are born helpless and unformed. In our distant past, as we evolved our big brains, women did not evolve big hips to go with them. Evolution faced a dilemma. Big-brained humans had a better chance at survival. Small-hipped women died in childbirth. Big-hipped women were too slow on their feet and couldn't escape predators. It was not the only possible solution by any means, but the one that happened was that women evolved slightly bigger hips and babies were born while they were still essentially fetuses. Thus a woman could protect her infant while it continued growing, now outside her body, but sheltered by the womb of her obsessive concern. And if a father could be persuaded to stick around, he would protect both the mother and her baby during this dangerous period. It was a rather clumsy, iffy and complicated solution, true, but evolution proceeds by barter and handshake, not by proclamation...
Our violent nature is what makes love possible. Totally peaceful creatures would not need the balm of love. Glance in the mirror, and a predator stares back at you. Prey animals - antelope, horses, cows, deer - have eyes located at the sides of the head, so that they can watch for danger, creeping up behind them. In contrast, the tiger has eyes facing front so that it can use its stereo vision to precisely pinpoint the whereabouts of the next meal, run it to the ground, and leap upon its neck with bared teeth. Humans have the eyes of a predator, which tells us something about our ancient origins. But we also have colossal brainpower. We are not just dangerous, we are ingenious. Without mechanisms for subduing our violent, craven and predatory appetites, we would have wiped ourselves out, adding our name to the long roll of extinction. But evolution gave us a powerful peacemaker. Our ability to love has saved us from ourselves."
Diane Ackerman in A Natural History of Love