Why can I only fall in love if I am absolutely positively sure that he can never return the feelings?

All of my major obsessions/crushes have been about guys I knew could never return feelings. My current obsession is a coworker who has a girlfriend and never notices me. My previous obsessions include a man who lived in Russia who was obviously too far away and a gay male in high school I used to be best friends with.

I am afraid of guys who show affection for me. Any type of major flirting or complimenting or asking to kiss puts me off. I feel like I am not ready to accept that type of behavior even though I am twenty years old because I feel obligated to return the feelings when I clearly don't. I also have fantasies I feel can only be carried out by certain people, and the people who are available don't seem to be a good fit for my fantasies.

Valentine's Day has got me wondering that I am way too pretty to be single, but I find no attraction to anyone that is too available and shows affection way too soon. I don't get why I am like this.