Having grit is probably the single most important trait of a founder, and one of the few which could reasonably argued to be innate. Just about everything else is learned, often by doing, which is why you need grit -- the learning curve is long and steep. Grit is how you bootstrap your way to being good at the other parts of building a startup.
Hard times create weak people. People aren't iron that gets tougher when forged in fire. People are bags of meat. People accumulate damage and damage creates scars. Scars are only "tough" in the sense that they are less easily damaged than the virgin tissue before. Otherwise, the function of the tissue is degraded in every other property.
Stress causes health issues. Particularly traumatic stress causes (wait for it) Post-Traumatic Stress Disorder. All else being equal, a person with ulcers and anxiety disorders and depression and PTSD is a less capable person than one without.
