Aka, how to place your ego aside and entertain.
Imagine, your players are out on an adventure and they meet up with a stuck up NPC who gives them a hard time. The mood is a bit tense at the table and the players may struggle to get what they want from this NPC. (And according to some, unnecessarily so.) Not a few moments later, they meet up with another NPC with a similar, if not down right same, demeanor. They may have different traits, but the feel of the NPC remains the same and seems to be a reflection of the DM’s attitude towards the players. Statistically this is highly unlikely to happen in real life but oh so common while playing a pen and paper RPG with other people. And naturally may lead to your players no longer having fun.