What makes you stop talking to a friend after 1 to 2 years of knowing them?

I made good friends in my masters program or I thought I did. After a big end of program trip a few people in my group of friends stopped talking to me and then 7 months later 2 more people stopped talking to me and now the final few that I was friends with stopped talking to me and I mean not answering my calls and messages some of whom I haven't spoken to in a while and some I was close to. When they do answer and I've asked for what happened and why they give vague answers about how they are going through personal stuff and life is busy yet I see them on social media meeting up with other people from our class. What gives? I am nervous about keeping in touch with even my acquaintances from my class now and don't want to meet with anyone else for this fear that if 10-12 people did this to me (mostly girls) the rest of the class will too. We graduated 2 years ago by the way and there were only 45 people in the entire class.

I feel so rejected and sad. Why did this happen to me? I went into this degree hoping to make life long friends and now I don't have any from there.

What makes you stop talking to a friend after 1 to 2 years of knowing them?
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