Premeditatio Malorum is a stoic exercise of expecting of terrible things to happen at any given time. I think for the most part anger came from an unrealistic view of a perfect world – that everything happens in our way. It came from setting up high expectation of the world such as everybody should have the same political views that we have, that all people are given fair treatment etc.
The Stoics believe that life is full of misery. There are going to be people begging in the street, government officials themselves breaking the law, some good people having deadly cancer etc. When people understand the ideal that suffering exists they expectations lower and not be disappointed with everyday life. This is how having low expectations minimize anger which is detrimental to your health and happiness.
