Expectations

I've previously learned this lesson a while ago - the hard way, and that is to NOT expect much from people. I've been heart broken so many times every time I expected something and it never happened, or when I expected someone to do something or to act or respond in a certain way and they never did! The problem is, " I expected too much" my mother and 'friends' often told me.
I have this friend who has this other friend and they disagree a lot. My friend complains a lot from her friend because her friend 'expects a lot from her.' That girl expects my friend to call her often, ask about her back... In short, she expects her to be the same kind of friend that she is with her.
When I heard this, I got the message instantaneously: I ought not be like that other girl, and so I keep my distance.
Then why do I get sad when my friend dumps me or ditches me for another? I wonder why I still do that sometimes! I want so much to be a good friend, and a best friend even but it seems so hard to be so sometimes.