That can and does happen to anyone, even during actual intercourse. It could be completely mental, such as not having the correct stimulus (try switching magazines :P), but it could also be caused by lubircant. Some, maybe most even, contain a retardent (lol) called 'benzocaine', which basically numbs your weiner so that you can last longer. Great if you're trying to impress some girl, stupid if your just whacking off.