I think that for some people, it's extremely difficult to stay friends with a person who rejected them romantically.. and I understand why this is sad. I have lost people in my life because we dated and it didn't work out.. This is a huge generalization but I've noticed differences in how women handle rejection, as opposed to men, based on upbringing. Women seem to remove themselves and feel down on themselves, while men tend to keep pursuing the object of their affection even if they got rejected.
I think this is a little bit off topic when it comes to the initial question, tho.
It seems a little odd to me to assume that everyone of the opposite gender you meet will be attracted to you, or you to them. Maybe I'm outside the norm, but I tend to develop a crush on someone not only if I sense we are compatible and I find the person attractive, but also if I get signals that the other person is also interested.