Sheldon has gone through a lot in Young Sheldon, but he didn't always learn the lessons from his experiences. The Big Bang Theory spin-off functions as the origins story for its most popular character — Jim Parsons' Sheldon Cooper. As a boy genius, growing up in Medford, Texas with his middle-class family wasn't easy. Oftentimes, people didn't really understand him, making it difficult for him to maintain a solid support system. While Mary and the rest of the clan have tried so hard, Sheldon still usually felt alone and isolated. That being said, just because he struggled, didn't mean that everyone else around him didn't.
Initially, Young Sheldon was focused on developing Sheldon's story. This relegated the rest of the family to supporting characters. As years went by, however, CBS realized the appeal of the Coopers and slowly increased the attention devoted to them. His family's narratives were more interesting in Young Sheldon season 6 than his. This doesn't mean that the boy genius didn't have some compelling arcs. He did, but it felt like CBS was clueless about what to do with Sheldon now that he is fully an adolescent. As he became noticeably more annoying and difficult, the show also highlighted just how many life lessons he didn't learn from his previous experiences.
Famously, Sheldon has always liked boasting about his intellect. One way he has done that was to work on projects alone. Had it not been for his Pasadena friends in The Big Bang Theory, he would have likely stayed aloof in Caltech. Thankfully, he had automatic collaborators because he was friends with Leonard, Howard, and Raj. Aside from them, he barely worked with anyone else, and in the rare times he did, he did it begrudgingly. Seeing how many times in Young Sheldon did he have to rely on other people for a project, it's disappointing that he never truly understood the power of collaboration.
While Sheldon has had the tendency to act like he preferred to be alone, it has been proven multiple times in both The Big Bang Theory and Young Sheldon that he actually needs people around. For years, he has complained about having to share a room with Missy, whining about how great it would be if he had his own space. When Missy moved out, however, he quickly realized that he actually liked having his sister around. Despite this realization, he has continued pushing people away, mostly by being insufferable.
Aside from his relationship with Missy, seeing Paige's downfall in Young Sheldon should have been an eye-opener for Sheldon. Unfortunately, he didn't seem to learn any valuable lesson from his fellow prodigy's struggles as someone who doesn't have a solid support system. He is usually always mean to his family in Young Sheldon and could be downright repulsive to his friends in The Big Bang Theory.
One of the biggest inconsistencies between Young Sheldon and The Big Bang Theory is their depiction of George. In the nerd-centric sitcom, Sheldon described his dad as drunk, and negligent. That being said, nothing compared to his revelation about him catching the Cooper patriarch in bed with another woman when he was 13 years old during spring break. There continues to be eager anticipation on how Young Sheldon will tackle the incident, but despite laying the groundwork for it, the prequel has been mostly ignoring the incident.
Regardless if CBS acknowledges the cheating incident in Young Sheldon, it has been clear that Sheldon never learned to fully appreciate his father and everything he has done for the family. In The Big Bang Theory, he only focused on all his flaws, failing to mention how George held his hand when he was scared during his first plane ride. He also never mentioned how his father tirelessly worked to provide for him and the rest of the family.
After being overlooked for years, Missy finally got fed up with the way she was being treated that she started rebelling. In her boldest move thus far, she attempted to run away from home to Florida by stealing George's truck in Young Sheldon season 6. She was eventually brought back home and the antic didn't solve anything, but Sheldon didn't seem to learn that the ploy wasn't effective.
In The Big Bang Theory season 7 finale, he pulled the same scheme as his sister a couple of decades earlier. What's interesting about his decision is that running away isn't really Sheldon's style. Couple that with the fact that Missy's daring road trip didn't get her issues resolved, and it's mind-boggling that her twin brother would think that it could solve his own woes.
Given his intellect, Sheldon often believed that he can do just about anything. So when he encountered a massive snag in his and Amy's bid for the Nobel Prize in Physics in The Big Bang Theory. The socially-inept genius really struggled with the failure because he supposedly never experienced failure like that before. That being said, Young Sheldon season 6 revealed that this wasn't the case. He had experienced something like this in his childhood when his controversial grant database failed. For some reason, Sheldon just didn't learn how to deal when things don't go his way healthily.
Before co-starring in the CBS family comedy, Sheldon's twin sister only appeared twice in The Big Bang Theory. In season 1, she visited Pasadena and spent some time with the socially-inept genius, and a decade later in season 11 to attend Sheldon and Amy's wedding. Despite her limited involvement, she quickly made an impression, and by the time she debuted in Young Sheldon, there was already interest in learning more about her. Eventually, it was revealed that Missy's childhood wasn't exactly fun despite her social skills. Growing up with a genius brother hampered her social life.
Still, Missy remained very patient and understanding of Sheldon. One too many times, the socially-inept genius sought her for emotional advice and guidance. That being said, Sheldon barely showed any appreciation for what Missy has done for him. When she visited Pasadena in The Big Bang Theory for the first time, he was critical of her. Even in Young Sheldon, he would often disregard Missy when he doesn't need anything from her. Perhaps Sheldon can make up for his attitude towards his sister, even just for a brief period of time in season 7.