why do men have premature ejaculation before sex?

Because then they wont need to get laid after all. The orgasm deal is done, they can clean up, take a shower, get dressed and call it a day........all without risky mating behavor!

Premature ejaculation is mostly a psychological problem and not physical. However Its better to consult a doctor becuase he has to figure out whether the reason is physical or psychological and treat you accordingly. No need to worry though. Its totally curable. And another thing is that inxperienced people when they first have intercourse might confront this problem. This is quite normal and not something to be worried about. So go see a doc and make sure if you really have the problem or not.