Warning: Spoilers for , season 5, episode 22!

Missy's (Raegan Revord) season 5 finale role explains why Sheldon (Jim Parsons) tolerated Penny (Kaley Cuoco) in . The CBS comedy wrapped up its year with an episode that shifts the focus back on its titular character. As Sheldon starts to fear the future, his concerns are eased by his twin sister, re-emphasizing how important their relationship is to his character development.

Since , George (Lance Barber) and Mary (Zoe Perry) have been mostly preoccupied with Georgie's (Montana Jordan) pregnancy situation with Mandy (Emily Osment). This leaves the Cooper twins largely on their own, and while Sheldon looks like he's indifferent to the whole family crisis, it is revealed that his home situation is starting to affect his studies. Ultimately, the constant fighting between his parents and Georgie's new role as an expecting father has caused him to develop a fear of the future. This is exacerbated by the appearance of his first pimple — a biological sign that he is growing up.

In the titled "A Clogged Pore, a Little Spanish and the Future," Sheldon's nagging worries finally becomes too much that he reaches out to Missy. Despite her initial reluctance to engage, Missy eventually helps her brother by assuring him that there's no need to be afraid of growing up and the future in general. The twins' conversation is both funny and heartfelt as they are forced to deal with their own issues while Mary and George tackle their financial woes, not to mention their relationship issues. This particular moment in the Young Sheldon season 5 finale explains why Sheldon tolerated Penny in The Big Bang Theory. Given her character in the geek-centric sitcom, Penny effectively reminded Sheldon of Missy; both aren't academically intellectual, but they are kind, spunky, and emotionally mature. With Missy not easily accessible to him, for the support that he used to get from his twin sister.

Looking back at The Big Bang Theory, it's easy to recognize how similar Sheldon's dynamic was with Penny to his relationship with Missy in Young Sheldon. In times of emotional duress, Sheldon usually reached out to Penny for some kind of advice or just a simple conversation. For what it's worth, Sheldon didn't have to mingle as much as he did with Penny even when Leonard (Johnny Galecki) dated her; he didn't get that close with Bernadette (Melissa Rauch) despite her marrying Howard (Simon Helberg). Admittedly, Sheldon was initially resistant to Penny, but after getting to know each other, he would seek her out for companionship. At the , Sheldon learned to genuinely love Penny. In fact, she may even be his most trusted friend as she was the only one who knew about George's cheating scandal. 

While Missy appeared on The Big Bang Theory, she didn't really have much interaction with Sheldon to show what kind of relationship they had. It's only in Young Sheldon that their dynamic is more fleshed out. Considering the impending tragedy of George's death, it's safe to assume that Missy and Sheldon will only be closer moving forward. No wonder when Sheldon got separated from his sister, he fostered the same kind of bond with Penny in Pasadena.