You want the short version. Okay, then: shit happens. Does that answer all your question? Thought not. Now… — Tom Holt

You poor suckers [mammals]: you've got all this horrible stuff to get through somehow so obviously there's got to be an incentive, or at least some kind of fix for you to get addicted to. That's love: possibly the sneakiest trick ever played on any variety of life form by a notoriously conniving Universe. You do all these dismal, soul-destroying things because you get attached to each other.