"That which does not kill me makes me stronger" is an often-quoted, perhaps the most often-quoted line from Nietzsche. I'm a big fan of Nietzsche, but I certainly don't agree with everything he said, and frankly, I'm surprised that he isn't called more often on this one. Clearly, what does not kill you still often leaves you worse off, weaker, not stronger.
The quote comes from Götzen- Dämmerung. Oder Wie man mit dem Hammer philosophiert (Twilight of the Idols, or, How to Philosophize With a Hammer), the chapter "Sprüche und Pfeile" ("Sayings and Arrows"), saying -- or arrow -- No. 8:
"Aus der Kriegsschule des Lebens. — Was mich nicht umbringt, macht mich stärker."
"From the war-school of life. - That which does not kill me makes me stronger."
War school. Nietzsche was one of those people who praise warriors without ever being one. He volunteered for duty in the Prussian army, but was not accepted as a soldier. The official reason given was "Schmallbruestigkeit," "narrow-chestedness." But even if Nietzsche had not been too skinny to be a soldier, he likely would've been too near-sighted. He also seems to have had migraine headaches. I have migraines. If you get them, imagine having them in the 19th century, without the benefit of any medications invented since then. Nietzsche had a lot of serious health problems his whole life long. It's widely speculated that his mental breakdown, which happened around the time Götzen-Dämmerung was published in 1889, was caused by syphilis. That's possible, but it could've been a lot of other things too. Nietzsche was a frail sickly guy, and eventually, in 1889, when he was 44 years old, something which did not immediately kill him made him completely lose his mind. From then until his death in 1900 he was unable to care for himself and mostly bed-ridden.
"That which does not kill me makes me stronger." Simply not true! No more true than "The meek will inherit the earth." Nietzsche was overcompensating. Maybe if he did not talk tough like that all the time, he would've broken down sooner. Or maybe he would've taken better care of himself and lived to a relatively robust ripe old age.
Watch out for pithy lines which sound good. So often we choose mottoes which sound good but don't really make sense.