There has been one constant throught all 16 Seasons of It's Always Sunny in Philadelphia and that is that the rest of the Gang drives Dennis to rage and fury. They frustrate him, badger him and make him explode with anger more often then not. It finally is all too much and Dennis needs a mental health day. Dennis has tried to leave the Gang behind before, once in order to become a "cool guy" again, and another time to be a father, but neither time has stuck. A mental health day seems like his only chance for sanity.
Source：https://www.cbr.com/its-always-sunny-dennis-technology/
