On Peace of Mind, X:

Knowing to what sorrows we were born, there is nothing for which Nature more deserves our thanks than for having invented habit as an alleviation of misfortune, which soon accustoms us to the severest evils. No one could hold out against misfortune if it permanently exercised the same force as at its first onset. 

From: The Ninth Book of the Dialogues of L. Annaeus Seneca, Addressed to Serenus. Minor Dialogs Together with the Dialog “On Clemency”; Translated by Aubrey Stewart. Bohn’s Classical Library Edition; London, George Bell and Sons, 1900; Scanned and digitized by Google from a copy maintained by the University of Virginia.