Whenever I complained to my dad about certain problems growing up, he’d either offer a potential solution or issue the stoic response: “it is what it is.” 
Those five words often felt uncomfortably complacent to me — what do you mean it is what it is? 
Some combination of teenage angst, naivete, and foolhardy optimism made the idea of a world filled with unresolvable problems totally unacceptable to me. 
It wouldn’t be until the last several years that I finally began to appreciate the wisdom behind his words — that there are two classes of problems in this world: the ones we can control and the ones we cannot. 
Important problems that are in your control are worth the energy; problems that are not in your control aren’t worth an ounce of it. 
Wisdom is being able to look at a problem, in much the same way that my dad did when I would present problems to him, and knowing which response is called for. 
Thanks, dad.