The nuSpock formula: Take a "Cage"-era Spock, more emotional and more vulnerable, and break him
. He was pushed over the edge, exposed to traumas the likes of which Spock Prime never experienced. It makes perfect sense to me.
And remember, the elder and much more stable Spock Prime was "emotionally compromised" too - his voice is breaking when he tells Kirk this, and at the end of the mind meld Kirk has tears pouring down his face and has to take a moment to compose himself. Why? "Emotional tranferrence is an effect of the mind meld" - those tears are Spocks, hidden behind his Vulcan stoicism.