"Don't start nothing - won't be nothing."
I apply this to dealing with people especially. If someone picks a fight with me, verbally or physically I respond in kind but with overkill. Sooner, rather than later, people don't start anything with me.
"Adversity breeds strength."
If shit is never difficult for you then you'll be weak. Coddling children emotionally, physically, and intelectually is how you create emotionally stunted, physically weak, and stupid adults. But much like smithing steel there's such a thing as too much heat.