What do you say to family/relatives/friends who ask why you're still single?

As we are now in October and the holidays are soon approaching, I'm sure a lot of us will get questions as to why we are still single or no longer with our significant other. How do you answer those who are pick-me's (female relatives especially) , bitter, smug or others who are genuinely curious? How do you react when your parents friends or maybe coworkers who say they can introduce you to their son/nephew/cousin etc?