It's getting increasingly clear through Young Sheldon that Sheldon's worst trait in The Big Bang Theory came from his mother, Mary. While it took years before the spin-off was greenlit, the Cooper family was already established characters in the franchise thanks to Sheldon's consistent stories about them. Eventually, The Big Bang Theory introduced them one-by-one as each family member, from Meemaw to Georgie, guest-starred on the original sitcom throughout its 12-year-run. Mary, however, had the most number of appearances, highlighting her strong relationship with his son.
Young Sheldon isn't the best when it comes to preserving The Big Bang Theory continuity. Aside from the string of plot holes it has created over the years, its depiction of the Coopers is the most explicit proof of this. Each member of the clan is varying degrees different from their original character descriptions in the parent series. George has always highlighted this problem as both Mary and Sheldon painted him as the bad guy in The Big Bang Theory when he's a totally fine, if not even great, dad in the spin-off. However, what Young Sheldon has always thrived in is focusing on the relationship among the family; Sheldon and Mary's close bond is a testament to this as their relationship is consistent in both series.
Missy correctly pointed this out in the Young Sheldon season 4 finale as she criticized her mom for always taking her brother's side. But aside from that issue, the episode titled "The Wild and Woolly World of Nonlinear Dynamics" also emphasized just how similar Sheldon and his mother are, but not in a good way. As the Cooper twins argued and George failed spectacularly to pacify them, Mary barged in, claiming that their household falls apart every time she's not in it. Earlier in the same outing, Mary attempted to take over for Pastor Jeff when he went home early; she thought that if she didn't, nothing would be done and that it was her purpose to step up to the plate because she's the only one who could do it. This kind of behavior makes Mary obnoxious and self-centered—exactly what Sheldon was when dealing with his friends in The Big Bang Theory.
While his Pasadena gang usually tolerated his other quirks and transgressions, Sheldon's regular berating was always a cause for serious fights; it was also why they didn't want to keep him around sometimes. At one point in The Big Bang Theory season 2, Leonard, Howard, and Raj decided to kick him out of their quiz bowl team because he was overbearing; this was despite knowing how much of an asset he would be on their side given his intelligence. There were several more instances like this over the years, but it was once again most prevalent in the series finale when Sheldon won the Nobel Prize in Physics. He insensitively demanded that everyone else's issues shouldn't be because their focus should be on celebrating him even though some of his friends had other personal concerns.
Mary always taking Sheldon's side likely contributed to his bad behavior in The Big Bang Theory. But it's also curious if the reason why she gave her son special treatment was that she saw herself in him. Since Mary tends to think highly of herself, something that's more explicit in Young Sheldon, perhaps she is letting Sheldon act mighty because she believes he deserves to. If this is the case, then perhaps Sheldon would've better learned about respecting others and their boundaries if he spent more time with Geroge despite his eventual infidelity.