I have been both in your position and the other side of this type of issue. I have to say, its not your fault either way.

When I find a person I fall in love with, I tend to shut out everything else out of my life even when I know I shouldn't (like that my friends are falling away, I'm not keeping up with them, etc etc), it usually even has a negative effect on my actual relationship I'm in as well.. I have to somewhat disagree with builderofcastles though, but in principle it's true as well.

The reason I have to disagree with his "feminism" as the cause of this is because I am bisexual and whether its a girl or guy I'm falling in love with, the result is the same, I completely ignore most of my friends because I don't want my partner leaving me or finding something better or feeling the way I felt when I was ignored for others by people I cared about with whoever I'm in a relationship with.. Its pretty unhealthy but like I said, its not really something I can help, semi-conscious types of decisions. I guess when I "fall in love" its more of an obsessive infatuation that doesn't really go away unless the other leaves me lol.. I'm not saying your friend is the same, but perhaps this is also playing a role in his life?

When my friends have done it it was painful as well. I don't know if I should have tried harder to remain friends when they "disappeared" or have said fuck em earlier tbh. I'm not sure if when I shun them after finding someone I'm crazy about has anything to do with a sense of revenge for others having done it to me in the past, hope not but obviously it's all part of a whole.. but even though I can empathise, it almost makes me even angrier because I can, and that when I was in their shoes, I had very little conscious appreciation of anyone besides "whoever it is that I'm in a relationship with", but as soon as the spell is gone, I realized my appreciation for them again, its terribly selfish, I know, and probably won't make you feel better, but it is honesty, maybe you have some thoughts on it to add for me &_&