Young Sheldon season 6 provides insight into why Mary was bitter on The Big Bang Theory. After facing difficulties and being fired from the church, Mary had to find a new job and deal with changes in her life. While the rest of her family's issues were resolved, Mary remained stuck in her new situation. The experiences portrayed in Young Sheldon explain her bitterness on TBBT. The article suggests that the character's development on the prequel spinoff is aimed at humbling her and providing context for her behavior on TBBT. Additionally, it mentions that Mary's storyline in Young Sheldon will eventually lead to the deterioration of her marriage with George and the upcoming dark plotlines.
