I guess it depends on your definition of "stronger". But, "stronger" isn't what I'd call it.

Every death I experience... every loss... makes me more sensitive towards it. It deepens my understanding of the process of life, death and rebirth. (Disregarding the subject of reincarnation, your body or it's ashes will serve another purpose as nutrition for continued growth and spawning of other organisms.) And it deepens my own understanding of myself and how I process these things internally. Is that stronger because I survived the experience and still continue on with my life process? Is that weaker because of how it has hurt me and how it will continue to hurt me every time I think about that particular person or animal that I have lost?
Personally, I think it makes each of us weaker. Every wound... every injury we sustain makes each of us weaker.

We walk with visible limps from physical injuries. We go through physical therapy just to try to get back to some semblance of normalcy. But, we'll never be back to 100% of what we were before the physical injury occurred. The scars and the loss of physical motion will always leave us with some weakness as a result.

We walk with invisible limps from our mental and emotional injuries. Those scars may be truly deeper than the physical ones. We chant, we meditate, we seek psychiatrists to give us the therapy we need to try to return to a normal life. Some turn to God but, we can't unsee the things we've seen. Some, it weakens emotionally to the point where they just can't deal with the pain and they end their own lives rather than continue. Some continue through the pain. Not because they are stronger because of the pain but, just because they haven't had enough pain yet to take them to that place. Or, because there are others that they do not wish to inflict that pain upon yet. And our deaths would inflict that. (Although, it will come to them eventually regardless of our efforts to stave it off.)

The visual and emotional Hell I suffered through as my aunt was dying from ALS was brutal.

Knowing that I was the only one she trusted to give her the correct dosage of morphine so she would not overdose several times each day just so she could suffer through her pain.

Watching the muscles of her useless body ripple under her skin as her brain sent signals they could no longer interpret.

Trying as hard as I could to make her laugh every time silent tears of embarrassment rolled down her cheeks because she needed to have me lift her, take her to the bathroom, hold her up and wipe her.

Her watching me BBQ for the family and then, knowing she could barely swallow liquids, indicated that she wanted to try the chicken that I cooked. I mashed a tiny portion of it between two spoons until it was damn near liquefied. And then, I held my breath and massaged her throat to make sure it went down. The pseudo-smile she was able to display was worth every skipped beat of my heart.

Then there were the Christians. They came night and day to pray for her. They brought more tears from her. Telling her there was One who could save her.

The worst part was the discussion with her (then) 17-year-old son. When she was still capable of speaking, she told me she wanted him to choose a nursing home for her care if that time ever came.

It came.

It came rapidly. It came with no consideration for her or anyone around her.

All of this happened in a 6 month span of time. My Aunt (who had always been the one full of energy) was gone in a period of time that went by so quickly.

It's been 20 years. But, moments of that time period stand still and they linger with me as if they haven't passed.

I was a pallbearer at her funeral. I carried her to the grave.

Every time I wrap my hand around a handle, I remember that.

She was 52 years old. I was 33.

So, no. In my humble opinion, what doesn't kill me doesn't make me stronger. It hurts like Hell.

But, what doesn't kill me... just isn't trying all that fucking hard.

I strongly disagree with this saying. There are too (please, notice how I know the difference between "to" and "too". I've seen the misuse of "to" TOO many times!) many people who are walking around with big issues due to circumstances that "didn't kill them" and those same circumstances didn't "make them stronger".

I know that my experiences with depression have helped me to be more mindful and empathetic of the suffering of others. I'm not sure whether or not it has made me stronger, but there has been at least a little positive come of it.

What doesn't kill us can cripple us. But we can gain different kinds of strength from different kinds of pain.

Maybe several individual hardships over a short period of time can cause death.

That depends on whether or not it's something from which we can learn a lesson, no? I mean, yes, if you survive a disease, you gain immunity to it, and that makes you stronger in a way; and the same goes for when you feed yourself small doses of poison in order to develop a resistance to it. But most of the time, the essential criterion is learning from what happened to you.

Yes and no. There are a lots of tough times in our lives where can look back and maybe indentify ways in which we have grown from that time. However, something like Dementia doesn't Kill you, and almost certainly doesn't make you stronger. 'what doesn't kill you will only make you stronger' is one of those sayings which sound good. But clearly isn't always entirely accurate

When we handle traumatic hardships gracefully. We often move to a higher energy level. If we don't, we weaken.

Does it make us stronger? Yes. We learn as we go through life.
Does it break us down to go through personal hardships? I think certain remarks can open old wounds that people haven't healed from. They have flash backs of abuse and hardship. Some people have post traumatic stress disorder on this forum and come here to get away from pain. Mean answers can cause harm to people and they stop coming here. My answer is yes, but that pain may open the door for them to search for a cure. Both questions can produce a positive effect in people. That's my take on these questions.

Unless it's an ongoing physical condition...I agree with @MsOliver and @resserts.

I think it's not literally true, as we can be physically weakened by illness or injury, but the figurative meaning holds true: if we aren't destroyed, the experience adds to who we are and enriches us. I think this is in line with what Nietzsche meant.

I almost drowned because I didn't know how to swim so I learned. I almost died in a car crash so I became a safer driver. I'm a troublemaker at times so I learned to keep my mouth shut.No doubt, stronger and wiser