gooberstain

So I have this friend who lives down here, only person I REALLY know who lives here, and he was one of my best friends for a long time (also happens to be an ex), but he hasn't called me once since I've been here and wont return my calls. I talked to his mom today because I promised her I'd call her, and she asked me if I'd talked to him at all, no. Told her I didn't know what I'd done to piss him off since I hadn't talked to him since the day when I was at her house, and found out he's doing all this bullshit just because he has a girlfriend. Why the fuck do people pull that shit? Just because you're bound doesn't mean you ditch out on those who got you through your fucking shit.
Anyone here understand why some people ditch out on their best friends EVERY time they end up with a girlfriend/boyfriend? If so, please explain this load of crap to me.
(People like that make me want to puke )