One of Sheldon's (Jim Parsons) unique quirks is knocking three times, but recently, he has mysteriously stopped doing it. While it can be just a dropped The Big Bang Theory trope, there might be a deeper reason why he no longer feels the need to annoyingly knock in counts of three. A couple of seasons ago, the show finally revealed the reason behind this habit. As it turns out, when Sheldon was 13, he came home early for spring break and accidentally stumbled on his dad having an affair with another woman. The two never spoke of it again, and considering how George Sr. (Lance Barber) died (not to mention Sheldon's close relationship with his mother), the incident has been understandably haunting him ever since.
This forced him to develop the habit of knocking three times first instead of just barging in on any door and risk the possibility of stumbling upon something that he has no business in seeing or more accurately, something that will negatively affect him emotionally. Because although Mary Cooper (Zoe Perry/Laurie Metcalf) was closer to Sheldon than his dad, Young Sheldon has so far revealed that the father and son have had their own tender moments. With such a deep reason behind this habit, why is it that Sheldon suddenly no longer does it? Making things more interesting is the fact that he actually didn't start doing it until the second season of the sitcom.
In the latest episode of The Big Bang Theory, long-time viewers have noticed that Sheldon just barged in the Hofstadters' apartment to talk to Leonard about the grant he has the responsibility to give away. However, it has actually been quite a while since Sheldon knocked on a door three times - the last of which was all the way from season 9's "The Geology Elevation" when he angrily went to confront Bert (Brian Edmund Posehn) in his office. Whether this is because there hasn't been as much opportunity for him to do so or simply because it's a dropped trope is unknown, but based on his behavioral pattern, there might be a deeper meaning why he seemingly stopped doing it.
A flashback scene that predates the beginning of the series saw Sheldon use his three knocks, but it wasn't until season 2's "The Euclid Alternative" that it actually officially debuted. At this point, Sheldon has grown accustomed to Penny being a constant figure in his life and shared more experiences with Leonard (Johnny Galecki), Howard (Simon Helberg) and Raj (Kunal Nayyar). He's developing genuine friendships, rather than just seeing them as minions.
Following the disappointment from Tam (Ryan Phuong/Robert Wu), Sheldon didn’t really care about anyone during the early days of The Big Bang Theory, thus no one was really capable of hurting him like his dad did. But when he started to get attached to his so-called friends, that's when he realized that caring for them means that they have an emotional effect on him. And to protect himself from the possibility of stumbling on something that his friends might be doing that could upset him, he started knocking three times on their, and eventually, everyone's doors.
So after years of doing this, why did he unceremoniously stopped? It's no secret that over the last couple of seasons, The Big Bang Theory has made a conscious effort to evolve Sheldon. While he still has some moments (trying to sabotage his wife's career), he's also exhibited progress - as proven by his generosity during his wedding with Amy (Mayim Bialik) and even on their honeymoon. By now, he's also learned to trust his friends enough to know that they will not do anything to intentionally hurt him. He's become more comfortable around them and not in a jerk-kind-of-way. This cancels out his fears of going through the same thing that happened with his dad. At this point, he's even closer to his friends than his own family given how they stuck with him all this years despite all his annoying traits and lack of social skills.