why do you get an orgasm from having sex? what causes it ?

Answer 1

It's a spasm of muscles in conjunction with nerve stimulation, usually in conjunction with ejaculation in males. In purely physical terms, it's part of the system that delivers sperm to the right spot at the right time. In women, the orgasm causes the pelvic muscles to contract, thrusting the opening of the cervix up and foward to create a seal, sort of funnelling the ejaculate in the direction it's intended to go.

In evolutionary terms, the orgasm may have come about as a form of positive feedback, a reward your body gives you for doing what is necessary to reproduce and propagate the species -- i.e. it feels good, so you're more likely to do it again, and often. Sort of the same reasoning why you seek out food that tastes good -- if it's sour or nasty, it's probably not good for you (back in the days before artificial sweeteners came along and made unhealthy things taste good -- and before we developed a taste for sour candies and atomic fireball gumballs -- fresh food tasted better than rotten food)