I have this as well. It has subsided over time. It was really bad when I was on Facebook when I would see people gush about their relationships.

I think for me the realization happened when I suddenly woke up and said to myself its okay to be alone. I know you said you didn't feel like dating and you were happy single, but were you really?

It will bother you, until you are really honest with yourself. Brutal but the truth. Also those dating sites for a quick bandaid for a relationship could cause more trouble than its worth. (Experience talking)

But I do hope you find peace of mind soon. Good luck!