Virtue gains much strength by being challenged
"our powers can never inspire in us implicit faith in ourselves except when many difficulties have confronted us on this side and on that, and have occasionally even come to close quarters with us. It is only in this way that the true spirit can be tested, – the spirit that will never consent to come under the jurisdiction of things external to ourselves. This is the touchstone of such a spirit; no prizefighter can go with high spirits into the strife if he has never been beaten black and blue; the only contestant who can confidently enter the lists is the man who has seen his own blood, who has felt his teeth rattle beneath his opponent's fist, who has been tripped and felt the full force of his adversary's charge, who has been downed in body but not in spirit, one who, as often as he falls, rises again with greater defiance than ever. So then, to keep up my figure, Fortune has often in the past got the upper hand of you, and yet you have not surrendered, but have leaped up and stood your ground still more eagerly. For virtue gains much strength by being challenged."
- Seneca, Letters 13.1-3