Love

Spoiler alert: If you are a hopeless romantic, or even a hopeful romantic, don't read this. I will expose the biochemical basis of love.

Sometimes you will see stories on popular media that say sex is pleasurable because it causes a release of dopamine in the brain. (That is not quite the right description. The dopamine release is the same as pleasure. It is like saying a building is tall because it has a lot of height.) They are only seeing a part of the story. Orgasm also causes the release of oxytocin (not to be confused with Oxycontin, a brand of time release oxycodone, a powerful pain reliever). Oxytocin is also released with nursing, intercourse, childbirth, and even hugging and touching.
Oxitocin has effects on the brain, causing feelings of trust, generosity, and bonding. In other words, love. The term "making love" is literally true! "Afterglow" may be the effect of oxitocin. If one partner withholds sex as a punishment, that could be very bad for the relationship. It might be better to have more sex.
For lots of technical details, and references, go to Wikipedia:
http://en.wikipedia.org/wiki/Oxytocin
or, the June 2008 issue of Scientific American.