I know I’m new here, so maybe I don’t understand the true set-up for these questions and responses, but…
I believe many of those who answered have misunderstood the saying. I take it to mean that life knocks you down. It doesn’t play fair. It hurts. It even stomps on you from time to time, or often. But if you can mentally, emotionally, physically, or spiritually get back up and stay in the game, you are stronger for it. Awful things have happened to me in the past, but because of having survived them, I can empathize with a friend going through something similar. That makes me a better friend. Then when the next blow comes along, I (hopefully) will be confident that I can take that one, too, and that it can force me to become a better person. Like steel sharpens steel…
[In other words, just what @netgrrl said.]