Well there's the basic obvious answer. Sex feels good. And who doesn't want to feel good? Sex in whatever form is also a distraction from whats going on around you. When you're focused on those feelings, whether with another, or self induced, either way, you're not thinking about money problems, your friend that stabbed you in the back, what a jerk your boss is, etc... your mind is cleared of all that junk, and you have good sensations going through you.
And if you're having sex with people, not only do you have that, but you also have that emotional connection. No matter how much you do to separate it, during the act, you can't help but feel close and a connection with that person. You aren't lonely for that time. And even if you are rolling solo, you pretend you're not alone. So i think thats a lot of why its so rampant.
Of course we also live in a society that teaches 'if it feels good do it'. So no ones going to think there needs to be any sort of moderation on such a thing.
