My dilemma

I'm in a questioning mode...
I've had lots of people asking me a rather similar question lately - or I should say, it's been like that for years and I get the question lots, but for some reason over the past week I’ve had this question like too much already, to where I'm really starting to wonder what the problem (or my problem) is. :)
What’s the difference between those that end up as singles and those that end up as couples (be it as boyfriend/girlfriend, partners, mates, married, etc.)?
It’s not looks, is it? I doubt that’s the case, I know tons of hot people (guys and girls alike) that are still single.
Is it how sexy they are? Nah…I don’t think so. - Sexyness pertains to singles and mated.
Is it because they have a flaw in their character? Again, I don’t think that’s the case as, and this isn’t meant to bash any couples, but I know plenty of people who are married that have immense flaws, albeit hidden by the eyes of love. (snicker)
So what is it?
It might be a personal choice for some so that’s not part of the dilemma here.
Of course, perhaps some really don’t care, or I should say, don’t put that much importance to it, seeing as there are other things more important in life than actually trying to find that one “perfect” one (did I make it too obvious that I fall under that category?) – but I’m really starting to find myself at a loss of words when asked that question. – Can someone help me out here?