content: "reason" is a tough word to use. The "reason" sex feels good is that a previous generation had a survival and replication advantage by enjoying sex compared to other humans who did not. Without knowing exactly what happened, we can't know for certain the reason it worked. Maybe it caused us to reproduce more, but maybe it led to greater pair bonding which led to a survival advantage.  Evolution is a sloppy thing. Something that had one "reason" may change into something completely different.