There could also be the following reasons:
1) Strong conflict within you. It is possible that, one part of you (consciousness) really wants to enjoy sex and get the wholesome satisfaction, while the other part (subconsciousness) feels the shame about sexual urge and sex itself (could be the result of overly strict rearing, where everything related with sex was depicted as bad and impure).
An unconscious compromise in this conflicting situation is the sex with premature ejaculation, which includes all those opposite things, mentioned above - you get some sexual satisfaction, but it is followed by the shame and confusion.
The solution in this situation could be the silencing of the voice, which lives in your head (usually this is the voice of parents, teachers and a society as a whole) and tells you to feel ashamed about sex and sexual urge. Instead, you have to make a lot louder your inner, authentic voice that tells you that sex is good, sex is healthy, sex is pure and you really want to enjoy it much longer and as much as possible.
2) The other reason could be the fear to lose the erection, therefore you do everything so intensely, that your excitement grows really fast. In this situation you have to calm down and learn to trust your body.