Source: Loretta G. Breuning

Do animals enjoy ? That question lit up the internet last weekend, but the right answer was overlooked by all the opinionators: sex can trigger both reward chemicals and pain chemicals, so it can feel good and bad at the same time. This answer is surprisingly relevant to us, because we have the same brain chemicals.

In case you didn't follow the action, Physicist Neil de Grasse Tyson innocently tweeted on the relevance of pleasure to natural selection, and was upbraided for forgetting animals with cruel and unusual parts and labor. A good time was had by all, but the neurochemical facts of life were completely left out.

The mammal brain releases chemicals that feel good when we do things that are good for the survival of our . The more relevant a behavior is to the survival of your genes, the more your body floods you with motivating happy chemicals, like , , serotonin and endorphin (in addition to the well-known sex ).

Bad feelings are released when the mammal brain sees a potential threat to its genetic survival. Cortisol causes the feeling we humans experience as , and , but the same chemical also alerts an organism to physical pain. The bad feeling of cortisol motivates a mammal to act urgently to make it stop.

opportunity is a mixed bag of rewards and threats, alas. I am not in any way suggesting that painful is natural or acceptable for humans. I am saying that our animal inheritance explains why romantic disappointments are so painful. In the animal world, the road to reproductive success is littered with unpleasantness, and genitalia evolve to keep pace with competitive survival pressures. But the good feelings of reproductive behavior motivate an organism to keep seeking a partner, even when pain is part of the package.

Source: Wikimedia Commons

Humans don’t have spiny corkscrew genitalia, and we don’t tolerate partners who try to eat us. Yet emotional pain is curiously persistent in human mating. Rejection feels like a survival threat, even when you consciously know there’s no dagger in your heart. Rejection triggers cortisol and kills your happy chemicals. It feels life threatening because from the perspective of your genes, it is. Even if you are not trying to have children, your neurochemistry was naturally selected to make you feel this way. A bad hair day triggers surprisingly strong neurochemistry because your brain links it to your mating potential.

The bad feeling of rejection has a good result. It wires your brain to stop seeking an unavailable partner, which frees you for a partner willing to do things that would promote your genes in the state of nature.

Lots more about your positive and negative chemistry in my post

When Love Brings Pain

my slide show

Why Love is a Neurochemical Roller Coaster

and my book

Habits of a Happy Brain.