reason for not accepting friend requests on facebook from people you know?

[:)]

I can understand if you don't want to accept a friend request from a stranger, but what if you know the person and they know you?
I added two girls on Facebook, one was a sister of my boyfriends friend who i added a couple days ago, and the other was my boyfriends friends girlfriend who i added a couple weeks ago. Ive been out with them both around 3 times now and every time they say hello to me and are nice and im nice to them and we have a good conversation. so it seems as if they like me by the way they act and talk to me, so why did they both deny my friend request?
What would be a reason why you wouldn't accept a friend request? When i go on their profile it says 'add friend' which means that they've denied it, it doesn't say something like 'friend request pending' or anything like that