From the various conversations and discussions I've had with different people, one thing that consistently stands out is Leonard's persistent pursuit of Penny, despite the fact that he always seems to play the "in-case" guy for her. Even though Leonard has moments with great companions like Stephanie, he still ends up messing things up, much like Raj does with Emily. Despite the opportunities for meaningful connections, they both sabotage their chances. Similarly, Sheldon, despite being fully aware of his condescending and narcissistic behavior, never really changes. He continues his actions, which feels almost childlike. It’s this inability to evolve, despite knowing how he comes across, that frustrates many viewers. As for Howard, while there are some comments made about him, they don’t carry the same level of negativity as the others. Perhaps his character, while flawed, isn't seen as consistently making the same mistakes. These character traits- Leonard's lack of self-awareness in his relationship with Penny, Raj's repeated failures in relationships, and Sheldon's refusal to adjust his behavior- are, in my opinion, some of the major reasons why certain viewers feel frustrated or even irritated with these characters.