The Young Sheldon season 6 finale makes Sheldon and Penny's best The Big Bang Theory moment so much more special. Sheldon's success in his adulthood was largely due to the support of his friends. While Leonard, Howard, and Raj were essentially forced to hang out with him after he saved them from an experiment gone awry, Sheldon's friendship with Penny evolved more organically. After she moved across the hall from Apartment 4A, and she subsequently started a relationship with Leonard, she also developed an odd attachment towards Sheldon, who wasn't exactly fun to be with.
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Over the years, Sheldon and Penny's friendship had become the best pairing in The Big Bang Theory — arguably even better than their respective romantic relationships. Sheldon and Penny had so many memorable moments in the nerd-centric sitcom, which is an intentional move from writers after realizing the chemistry between Kaley Cuoco and Jim Parsons. Some of their best interactions, however, were in the earlier years of The Big Bang Theory while they were still figuring out each other. Their biggest friendship breakthrough happened in season 3, which the Young Sheldon season 6 finale significantly improves on.
As George and Missy see Sheldon and Mary off to Germany in the Young Sheldon season 6 finale, the Cooper twins remain in conflict. Despite Sheldon's previous attempts to apologize to his sister about ratting her out for sneaking out at night, she remains mad at him. When she refuses to hug him before he leaves, Sheldon claims that he doesn't care for them anyway. Seeing him not wanting to do that to appease Missy and then eventually offering one to Penny in The Big Bang Theory season 3 after she gifted him a Leonard Nimoy napkin makes their interaction so much more special.
Interestingly, however, parallels between Missy and Penny are not lost on viewers. Somehow, both characters have the same function, just at different points in Sheldon's life. Missy offers emotional guidance and support to her twin whenever he struggles in Young Sheldon. Meanwhile, Penny did the same thing for him in The Big Bang Theory. Both Penny and Missy aren't also afraid to call out Sheldon when he crosses the line, setting him straight whenever he becomes too rude or mean.
Looking back at The Big Bang Theory, Sheldon's uncharacteristic tolerance of Penny maybe because she reminded him of the strong women in his life. While Penny is usually compared to Missy, there are also traces of Meemaw in her when it comes to being frank, joyful, and feisty as seen in Young Sheldon. Seeing Sheldon slowly opening up to Penny in The Big Bang Theory's 12 seasons was probably one of the most satisfying things in the show. Their hug back in season 3 was memorable, but they have had more of them in the succeeding years.