It's not because of the way they treated me. It's not because of they way they treated my daughters. I don't like my ILs because of the way they treated my husband. I can get over immaturity, insecurity-based lash outs, games and manipulations directed at me; that, I could and did forgive and mostly forget. They're basically fools and so I was able to expect not much more than foolishness from them. But as long as my husband is hurt by his parents, I hurt for him. I can forgive what's done to me...but for me it's much harder for me to forgive what's done to someone I love so much.
Yes they raised the man I love. But I saw for myself how they poorly treated him. After I learned just how much their actions didn't just annoy but hurt the love of my life, and that he was done with them, I felt I could be done with them. Maybe it's overblown loyalty.
Mom2 made me think when she posted about why she doesn't like her ODD's BoyFriend. It's because ODD told Mom2 how the guy did some awful things to ODD. He cheated on her, he wasn't there for her when she was ill enough to be hospitalized, he sexts with other women, etc. Mom2's ODD has forgiven and forgotten what her own BF did to her, and now expects her Mom2 to do the same. And of course Mom2 doesn't have the good times with ODD's BF that ODD has, in order to build back the good will towards him.
I wonder how many DS' told their wives (DILs) some horror or "horror" stories from their childhood that later affected the DIL-MIL relationship. Tales that range from outright abuse to what the DIL might see as neglect (including medical neglect) to plain old perceived sibling favoritism. Exchanging childhood stories is something new couples do. And the stories may keep coming, especially after the couple has their own children togther. But before the girlfriend/DIL even meets her future ILs, she may resent what she thinks they did "to" the man she loves. And she doesn't have the rest of the good memories the DS does to balance the whole picture.
And while the DS may have completely forgiven his parents for the ancient stories he shares with his new girlfriend/wife/mother of his children and come to the place where he accepts 'they did the best they could with what they had at the time", the girlfriend/DIL/new mother upon hearig the stories might not be able to let go so easily. Not for years, if ever. And if DIL is suffering from the mess, she may never forgive her MIL. I know an otherwise sweet woman (a MIL herself) who got along well enough with her MIL - until her husband got skin cancers. Now she openly and rather viciously blames her MIL because she didn't protect him from childhood sunburns. (Interestingly she still likes her FIL, who also failed to protect him from the sun). Every chance she gets: "My MIL gave my husband skin cancer." She also gave him life, but apparently that doesn't count.
Seems to me a DS story that goes "my dad beat me" can be instantly translated into "his mom didn't protect him from being beat." And that MIL may never catch a break from that DIL. Even a common story like "My mom thought my younger brother could do no wrong so I got punished every time he did something." or what could be a completely subjective story like "I was always compared to my older brother and reminded how I never measured up to him." Or one I heard recently "My parents paid for my sisters college and ran out of money by the time I was in school so I'm the only one in student debt." Those could tap into a sibling-rivalry issue the DIL has of her own, or just seem so wrong to the DIL that it may take years of seeing for herself that the MIL does love and is kind to her son, for DIL to be able to see the MIL with clear eyes.
It may not be fair. But it happens and I wonder how often it happens. When a DIL is beachy to everyone, she is may be just a beach. But when an otherwise decent woman of a DIL seems to target her husband's parents out of the gate, parents who are perfectly nice people and do treat her well enough, it may be for reasons other than her insecurity or selfishness or her just being a DILFH. I wonder how often it might be a certain sense of "loyalty" and old pillow talk coming back to haunt.