I suspect in my scenario it was his lack of having kids and the ability to do so (they are not able to have kids). Best I can tell is it was painful for him to see / hear about my life with kids now.
Because our relationship had become long distance it mostly centered around long phone calls or gaming / vr sessions together ever month or so. I basically stopped trying to schedule these interactions because it became clear it was having such a negative impact on his emotions. I initially tried to avoid talking at all about kids and all that but it didn’t seem to help the situation.
Anyway that’s what I just went through and at this point I don’t know if we’ll ever connect again, it’s been 6 months since we exchanged emails, which used to be daily / multiple exchanges a week.