I don't know where all this "only massive suffering builds strength" stuff comes from.
I'm pretty sure this is analogous to the way you build up muscle: you don't try to squat 300 lbs, tear your hip flexors, and destroy your knees on day one. In fact, repeated injury makes you weaker.
Most social groups do have a mechanism to softly introduce low intensity conflict as play (which may help with emotional strength). Practically any group has escalating banter with escalating intimacy, for instance, permitting growth of emotional resilience in a progressive manner.
I was never bullied in school and I'm generally quite socially comfortable in many situations.
