Warning! SPOILERS for Young Sheldon season 5, episode 4
Young Sheldon season 5 explains Sheldon's (Jim Parsons) complicated reaction to moving with Amy (Mayim Bialik) in The Big Bang Theory. After five years of proper dating, the socially-inept genius and her girlfriend finally decided to live under one roof. However, this wasn't an easy decision, especially for Sheldon, who had to be convinced by Penny (Kaley Cuoco) and Leonard (Johnny Galecki) before he would agree. Even then, he was still hesitant, and his reaction could be attributed to his experience when he was still living with his family in Texas.
In Young Sheldon season 5, episode 4 titled "Pish Posh and a Secret Back Room," Georgie (Montana Jordana) was able to finally convince his parents that it's best for him to drop out of school. However, since he'd be working full time, he'd have to pay rent and utilities to his parents. Aside from that, he'd also have to move into the garage so he gets some sense of his newfound independence. Missy (Raegan Revord) took advantage of the situation and suggested that she takes over his big brother's vacated room. She reasoned that she and Sheldon (Iain Armitage) can't really share a bedroom anymore since they are growing older and would need their respective privacies. Convincing her parents was easy, but things were a little different with her twin brother.
Sheldon was scared, but he wouldn't admit it. Even after he got all the space for his toy trains and science equipment, he was still unhappy with the new living arrangement. It took him quite a while and a few times knocking on Missy's door at night to settle down. George (Lance Barber) and Mary (Zoe Perry) also had to give the twins walkie-talkies so he could reach out to her whenever he gets lonely at night. Having gone through this, it's understandable why Big Bang Theory's adult Sheldon wasn't simply thrilled with the idea of him and Amy moving in together. Instead, he was “excited, concerned, a little scared.” It can be remembered that when they hashed out that plan in The Big Bang Theory, it was supposed to be a temporary setup. Amy's apartment was flooded and she needed accommodations for five weeks. After that, they'd get back to their old set-up. Considering how hard it was for him to get used to Missy getting her own room, Sheldon was afraid that it’s also going to be difficult for him when Amy moved out.
As later revealed in The Big Bang Theory, Amy's apartment repair finished early, but she didn't say anything about it because she was having a lot of fun spending more time with her boyfriend. It caused a brief conflict that even got Penny and Leonard involved. In the end, they all let it go, because Sheldon wasn't really opposed to keeping their current living arrangements anyway. He just didn't like the fact that he was being duped when, in reality, he didn't need any more convincing that he and Amy should continue rooming together. Living under the same room may have actually also inspired Sheldon to get married. Granted that it was an unsolicited kiss from a colleague that ultimately forced him to propose, Sheldon had to realize how much fun his life has been since he and Amy moved in together to motivate him to finally take their relationship to the next level.
With Sheldon and Missy sleeping in separate bedrooms, Young Sheldon can now progress their character arcs removed from each other. This is especially important for Missy, who had just gone through her first heartbreak. Considering her relationship woes in The Big Bang Theory, expect her to be more active in the dating scene than her twin.