In my particular case it was that in hindsight I don't think I was as close to a lot of those people as I thought I was at the time. When I moved away, many of

In my particular case it was that in hindsight I don't think I was as close to a lot of those people as I thought I was at the time. When I moved away, many of them didn't even really say goodbye or goodluck or anything like that. I kept in touch with a few of them for a couple of years after I left, visited a couple of times. Then they got married, had kids, and I hear from them much less. My understanding is that many childless people have a similar experience where their friends who marry and have kids move on from them fairly soon after, as their focus moves more onto their kids