Well I do know that my boomer dad has never forgiven his mom for having a second child. His dad was an abusive ass, but for some reason he's more mad at his mom, because she "was taken away from me when I was only a year and a half old", by which he means his mom had to split her attention between him and his newborn younger sister.
I don't know how this fits into what you're saying, probably it doesn't, but I do think it's notable that even though my dad hated his abusive piece of shit dad (for good reason!), somehow it's the sadness that every single oldest child has to deal with (including me, fun fact) that has, apparently, caused my dad so much anguish over the course of his life.
I have a hard time empathizing with my father. He truly seems to believe he's the only human on earth who has ever suffered emotionally. Very frustrating to try to talk to him as a fellow human being, because he sure doesn't see me (or anyone else) as one!