I suspect, like many human things, it's an underlying biological drive interpreted by our lawyer-as-narrator, the consciousness. I'm thinking of the "elephant and the rider" metaphor -- the elephant has drives for sex and care of young, the rider on the top rationalizes it.

The elephant turns left and goes into a river. After they're headed for the river, the rider thinks, "I'm hot, I should go to the river."

Sex and caring for young directly results in "being fruitful and multiplying".