Marcus Aurelius said;

The happiness of your life depends on the quality of your thoughts. You have power over your mind – not over outside events. Realize this, and you will find strength. Dwell on the beauty of life. Watch the stars and see yourself running with them.

Seneca said :

For what prevents us from saying that the happy life is to have a mind that is free, lofty, fearless and steadfast - a mind that is placed beyond the reach of fear, beyond the reach of desire, that counts virtue the only good, baseness the only evil, and all else but a worthless mass of things, which come and go without increasing or diminishing the highest good, and neither subtract any part from the happy life nor add any part to it?

A man thus grounded must, whether he wills or not, necessarily be attended by constant cheerfulness and a joy that is deep and issues from deep within, since he finds delight in his own resources, and desires no joys greater than his inner joys.