Cause staggering amounts of dread despair or deep, deep joy.
But how can mere chemical interaction breed super massive want/attraction?
Making nonesense out of sense;
Obsessive fascination beyond explanation.
What's it for?
Somewhere between the snake and the apple...desire happened. So its a perk of being a human being. Wanting, aspiring, desiring. It's not weakness, it's experience. It gives us more that it takes from us.