When he is not confident enough to think that he'll be able to trust a partner; When he would rather deal with his life's finances and get his career going before choosing to try that sort of thing; When he is fulfilled in life, and just doesn't think there could be any more room (Although I'd disagree with this). There are other circumstances, but they are far in between, I'd say.
Maybe you should ask yourself, "Why NOT remain a bachelor?" Then depending on what you come up with (which could be nothing!), you'll be surer of the answer.