What causes sadness immediately after sex?

Rebound in the brain: A problem like this happens for a number of people. While we don't yet understand why, here's an idea provided by richard friedman, md: the sadness (sometimes referred to as "post-coital blues") could be a strong rebound effect in the brain after orgasm that makes some people feel sad. Ssri medications tend to decrease the intense pleasure of sex. They also may eliminate the post-coital blues.