I hate, hate, hate this question! It implies that being single is like an illness and they want to find out what causes it. "Yes, he is very beautiful but he is high-maintenance", "Yes, he is handsome but he is dumb" (that's another pet peeve of mine but I won't go into that now). Stop trying to find out what the reason is for someone's single status. It is rude and next time you ask me that, I won't answer and I will make this face instead.