Warning! Spoilers ahead for Young Sheldon season 5.
Inadvertently, Young Sheldon's 100th episode explains Mary and Missy's final fight in The Big Bang Theory. It is no secret that the mother and daughter don't have the best relationship in The Big Bang Theory's prequel. But as fans of The Big Bang Theory know, their contentious relationship continues moving forward as they bicker in the nerd-centric sitcom.
Mary and Missy were the first two members of the Cooper family who appeared on The Big Bang Theory, both making a cameo in season 1. But while Mary appeared several more times in the next few years, Missy didn't show up until a decade later when the mother and daughter came back to Pasadena together. At that point, they were going to attend Sheldon (Jim Parsons) and Amy's (Mayim Bialik) wedding. But while it was supposed to be a happy occasion for the Coopers, there was some family drama, including a car argument between Mary and Missy.
As Raj (Kunal Nayyar) drove with the pair from the airport, Missy revealed that even while she's pregnant with their second kid, she's in the process of separating from her husband. Mary, however, sounded in denial of this reality. At first, she tried to lie about the real status of her daughter's marriage which irritated Missy. It didn't become a full-blown fight but Young Sheldon's 100th episode offers an explanation for the Cooper matriarch's behavior. In the episode titled "A Solo Peanut, a Social Butterfly and the Truth," Mary refused to accept the fact that some guys "suck" as Meemaw (Annie Potts) put it when complaining about her break-up with Dale (Craig T. Nelson). Missy was on her grandmother's side, which irritated Mary more. This highlighted that she’s more than happy to pretend things are going well in a relationship than face the facts.
Somehow, Mary's refusal to acknowledge the ugly parts of life can also be linked to the downfall of her marriage. George (Lance Barber) has been clearly having a difficult time even before he encountered problems at his job at Medford High School, but Mary seems to be oblivious about it. In fact, even when her husband opened up about it, her first instinct was to invalidate his feelings. While it's unfair to pin the breakdown of the Coopers’ marriage all on Mary since George did cheat, as confirmed on The Big Bang Theory, it's been proven time and time and again that she has the tendency to sweep difficult things under the rug — at least, except when Sheldon is involved. But even then, instead of facing the difficulty of a situation, she tends to shy away from it, perhaps hoping that it will resolve itself.
Young Sheldon has made it clear that Missy has a better relationship with George. This helps to explain why she never bashed her dad when she appeared on The Big Bang Theory like Sheldon and their mother was inclined to do. Not much is known about how she copes when the Cooper patriarch dies, but chances are that it only makes her dynamic with Mary more complicated.
Young Sheldon season 5 airs Thursdays on CBS.