Girlfriend Status?

Ok. So the theme today is "Guys you dated that you did nothing but be nice to then they dump you for no reason, but you are cool about it and want to stay friends with them but they hate you for no reason." This conversation continually comes up between my girlfriends and I. Lately, there is has been an increase is this phenomenon.
Why does this happen? Do guys get upset when we DON'T freak out, go crazy or stalk them after a break up?
Is it immaturity that keeps men from being friends with girls from their past? Isn't that the purpose of "dating"- getting to know someone to see if you want a future with this person someday? So, why should anyone get pissed off if it just isn't working out?

Oh, and while we are on this topic--
If I only dated you for a few weeks/ months it DOES not mean I thought you were my boyfriend and I may not even have been that into you anyway. You don't have to ignore me after you give me the "you don't want a girlfriend speech". I wasn't trying to be your girlfriend. When I list the traits I am looking for in a person that I see myself "dating" someday; spoiled, self-loathing, self-centered, ungenerous, ungreatful, and asshole aren't usually on there.
It works both ways, sometimes we aren't that excited about you either. For instance, when you pretend the green onion on your sushi plate is a joint, and then you pretend smoke it and pass it to the people at the table next to ours, I know we do not have a future together. Therefore, I am in no way trying to be your girlfriend. I'm just along for the ride until a better train rolls thru the station. BUT, even though I was completely appalled when you refused to wear shoes into a restuarant and you threw a child-like temper trantrum on the parking lot for no reason- I do still want to be your friend...because when you aren't bitching and complaining about how bad your life sucks, you are kind of fun.