It is a universal law — have no illusion — that every creature alive is attached to nothing so much as to its own self-interest. - Epictetus

People are not disturbed by things themselves, but by the views they take of those things.

You should be especially careful when associating with one of your former friends or acquaintances not to sink to their level; otherwise you will lose yourself. If you are troubled by the idea that ‘He’ll think I’m boring and won’t treat me the way he used to,’ remember that everything comes at a price. It isn’t possible to change your behavior and still be the same person you were before.

I am richer than you, therefore my wealth is superior to yours’; and ‘I am a better speaker, therefore my diction is better than yours.’ But you are neither wealth nor diction.