Don't know for sure, but the following might have something to do with it --
I'm not real good at staying in touch. I don't get on the phone to someone unless I have something significant to share -- almost never just to chat. So I don't call much. But yet they don't make any effort to call either, so I guess we are actually both at fault here.
I don't invite people to my place because I'm a slob. Or maybe I'm a slob so I don't have to invite people to my place. Regardless, I kinda have this feelling of violation when someone comes inside my house. So I never have parties or invite people over for dinner.
Once they marry and/or have children, I lose interest in being with them. Kids get in the way both physically and emotionally. And spouses are always putting rules and limits on my friends' time with me -- there's always a reason why my friend has to get home early. I have no limits and can stay gone as long as my heart desires, and I don't like that I have to end my fun because of some jealous spoiled brat husband.
I get tired of participating in things my friends like to do, but they never extend the same courtesy to me -- never show any interest in things I enjoy. Eventually I just think "I'm doing things by myself anyway. Why bother asking friends to come along any more?"
There's probably more -- I'm opinionated, I talk a lot, I'm a bit of a know-it-all, I don't drink or smoke so some consider me dull. But I think these are the most likely reasons.