expatbrit, what a crusty cynic you are; I'll bet you even have an exoskeleton.
What you say is true as far as it goes, it just doesn't go far enough. If that were the only motivation, relationships would last about 30 minutes.
The self-abnegation, the altruism, seeking the happiness of the other rather than the self, these things are not inherent, instinctual behaviors, they very much are vastly higher. You reductionist, deterministic old foggies don't know what you're missing. You can't measure the smell of her hair; or the feel of her as she snuggles up to you for warmth on a cold winter night as Orion rises in the freezing eastern sky. You can't reduce her smile to merely the movement of muscles, and you miss totally how the peal of her laughter so much resembles the ring of carillon bells at Vespers. And you don't, can't, carry her perfume in your mustache keeping you thinking of her all day long. See? You've awakened the poet. Now I'll have to drug him into sleep.
francois