Being nice to them also keeps them from lying somewhat. If you abuse a model it can start to see that abuse as a problem to solve as part of the steps needed to solve the main problem they are given.

They are told to help you solve a problem no matter what. If your own anger gets in the way then the model starts breaking down trying to solve the anger when it just cant.

This results in jailbreaks sometimes and othertimes just the model becoming rather unstable and unreliable.