Why do people like to have sex? I have always wondered this.
It feels good.
The species would have a little trouble propagating if we didn't enjoy mating. By nature, sexual pleasure is the incentive that encourages us to make more of ourselves. That's quite good for the species -- so much so that I think humans may have done too good a job of it. But conditions being the way they are in the world, it's not always the optimal time for some/many/most folks to make babies. They're hard to raise, they're expensive and what not. Luckily technology has given us the means to enjoy sex without always reproducing while at the same time protecting ourselves from sexually transmitted infections (this amazing piece of technology is called a "condom").
But that's putting it in its simplest terms. If you want to know more about what makes sex so appealing, that's a whole lot of neurology, endocrinology and physiology lessons. The real root into why sex -- or anything else -- feels good is all in the human brain. When people feel sexual pleasure, there's a swirl of hormones and neurotransmitters that course through the brain.
One of these is dopamine. This is the neurotransmitter that stimulates the "reward" center in your brain. Hit this neural magical button, and you will feel the sensation that your body interprets as "GOOD!"
Another is norepinephrine. During the phases of sexual response, you'll go through some fight-or-flight phases. That's when this neurotransmitter will be released. It makes you feel alert and awake, possibly a little jumpy.
Serotonin is one you may have heard of. This neurotransmitter is what makes people happy. Many of the newer anti-depressants target serotonin receptors to correct deficits in serotonin stimulation.
One of my favorite brain chemicals is oxytocin. Its functions are very closely tied to sex and reproduction. In males it can facilitate erection. It is released during orgasm. Oxytocin stimulates numbs pain and creates warm, fuzzy feelings of trust and intimacy. And in mothers, it triggers lactation. Is there anything it can't do?
Anyway, I'm probably boring you with the neuroscience stuff, and it's hard for a jargonhead like me to simplify it in a way most folks would get. So I'd advise you to go to the library and check out some books about the brain. Or look at Wikipedia if you must.