And what is this Good? It is a clear and flawless mind, which rivals that of God, raised far above mortal concerns, and counting nothing of its own to be outside itself.
You are a reasoning animal. What Good, then, lies within you? Perfect reason. Are you willing to develop this to its farthest limits — to its greatest degree of increase? Only consider yourself happy when all your joys are born of reason, and when — having marked all the objects which men clutch at, or pray for, or watch over — you find nothing which you will desire; mind, I do not say prefer.

Here is a short rule by which to measure yourself, and by the test of which you may feel that you have reached perfection:
“You will come to your own when you shall understand that those whom the world calls fortunate are really the most unfortunate of all.

~ Seneca, On the True Good as Attained by Reason

No man is good by chance. Virtue is something which must be learned. Pleasure is low, petty, to be deemed worthless, shared even by dumb animals — the tiniest and meanest of whom fly towards pleasure. Glory is an empty and fleeting thing, lighter than air. Poverty is an evil to no man unless he kick against the goads. Death is not an evil; why need you ask? Death alone is the equal privilege of mankind.

~ Seneca, On the Conflict between Pleasure and Virtue

You may judge it the most grievous of ills to lose any of those you love; while all the same this would be no less foolish than weeping because the trees which charm your eye and adorn your home lose their foliage.
Regard everything that pleases you as if it were a flourishing plant; make the most of it while it is in leaf, for different plants at different seasons must fall and die. But just as the loss of leaves is a light thing, because they are born afresh, so it is with the loss of those whom you love and regard as the delight of your life; for they can be replaced even though they cannot be born afresh.

“New friends, however, will not be the same.” No, nor will you yourself remain the same; you change with every day and every hour. But in other men you more readily see what time plunders; in your own case the change is hidden, because it will not take place visibly. Others are snatched from sight; we ourselves are being stealthily filched away from ourselves.
You will not think about any of these problems, nor will you apply remedies to these wounds. You will of your own volition be sowing a crop of trouble by alternate hoping and despairing. If you are wise, mingle these two elements: do not hope without despair, or despair without hope.

~ Seneca, On Care of Health and Peace of Mind