I think that there may different reasons for different people - us humans are a complicated bunch and I don't think there will ever be a black and white answer to this question.

I can say from my own personal point of view that I had a one-nighter once. I'd found out that my then fiance had been having an affair and I'd dumped him. I felt low, not sexy, unwanted and not good enough, and I had a one-night stand with a bloke who I knew liked me from the way he looked at me whenever I saw him in the pub.