Why do people do this on Facebook?

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Ok so what happened was that I sent a friend request to one of my classmates 3 years ago when I just discovered Facebook. I've known him since Elementary School and I'm sure he was online often but he never accepted my friend request. I also see him at least every week in school since that day but I never mentioned a word about "Facebook".

So today, he accepted my friend request and I'm sure that he accepted many friend requests in between that time.

My question is why do people not accept friend requests but accept them like many months later? This isn't the only person that did that to me. I remember that there were like 20 other classmates that did that to me.

What is the psychology behind all this? Has it ever happened to you before or have you done that to people?
this place has become more retarded...........