Great Answer by Kaushik Parashar on Quora:
Love is a chemical state of the mind that sustains harmony and drives life on planet earth forward. We’re wired to be romantic creatures. Our brains make us some awesome cocktails with which we’re rewarded to move through various types and stages of love.
First cocktail: Dopamine (Pleasure chemical) mixed with Norepinephrine (racing heart, excitement chemical), phenylethylamine and reducing levels of serotonin (causing obsession). After estrogen and testosterone get our sex drive area active (The lust part of love story), we start looking. When the potential partner (We need not know it consciously, but we have a flexible love template affected by our genes and upbringing) is found and attraction grows, we’re being rewarded by this cocktail. We get so high that we’re almost blind, giddy and can’t think of anything but romance. And that’s pretty wonderful because this is the biological drive that helps us focus on that one person and the relationship. The longer this “Lovin’ feeling”, the better are chances of the relationship to survive.
Second cocktail: Oxytocin mixed with vasopressin and endorphins. Our brain decides its time for us to get out of fantasy love high and taste some real love. Oxytocin and vasopressin interfere with our dopamine and norepinephrine, and we suddenly find that the “Lovin’ feeling” vanishes. Actually we’re just getting real and this cocktail now creates an emotional bond. (Creatures who don’t have the receptors for these, are the one-night-stand creatures… so human babies are a bit fortunate) The more sex, the stronger becomes the bond. As we keep getting rewarded with this cocktail, attachment grows and we have a general sense of well being. We feel soothed, peaceful and secured. And this is pretty wonderful because the oxytocin in the cocktail drives mother-infant love, uterine contractions for childbirth and the needed reflexes for breast feeding.
So when we’re just having fun with our cocktails of love, there is this whole lot of stuff going on behind that tries to sustain our wonderful lives on planet earth. Such is love.