Why are guys rude/less interested in you/jerks when they realize you aren't interested?

Alright, well this question has been on my mind for quite a while. I tend to have a harder time than my girlfriends at having male friends. For example I'll meet a guy that's started hanging out with my group of friends.. I'm a friendly person, a good listener and am always interested in getting to know new people, but after a little bit when they find out I'm not interested in dating or that I'm in a relationship, whatever the case may be, all of a sudden I'm pretty much non existent, I'll attempt to have conversations or general chit chat to be friendly but it always seems like it's too much of an effort for any of these guys to maintain some form of friendship. Another example is a guy I used to be pretty tight with, nowadays he only talks to me when I start the convo or catches up when I suggest it. My question is why? Why can't/don't guys want to have a friendship with me? Or why do they put such little effort, after seeming to be interested in getting to know me, then just not at all after? I even had one guy who I spoke to on Facebook after being pretty tight (used to talk on the phone a fair bit), that he was surprised I spoke to him, but he seemed so uninterested in a convo, then when I saw him out next he said 'do talk to me again on FB next time'. Ummm WTF? It seems to happen so often. If anyone can shed some light I'd be grateful.
Why are guys rude/less interested in you/jerks when they realize you aren't interested?
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