system
Given the following post, respond as if you're commenting on Reddit:
human
Why does rough childhood makes some people strong while some mentally ill?

Are people just randomly born weak and strong?  
*edit: strong is keeping one's sanity at full health despite possibly-traumatic events, weak is getting ptsd or other illnesses because of it*
gpt
Depends on Natural resilience, time span and onset of bad experiences (earlier and longer being worse), existence of a social support net or care taking alternative, the kind of instability (family, societal, emotional/financial/...). 
In short: there are a lot of factors.