A mind-meld works both ways...
A dare. Spock had almost laughed at that: James T. Kirk had joined Starfleet because Christopher Pike dared him to. It was absurd. Who would have thought that the Kirk of this timeline would be so very different to his own? And yet, of course, he was similar enough that Pike had issued the dare: the older captain had clearly seen something special in the young man in order to put his reputation on the line by recommending the troublemaker to Starfleet Academy. Spock could very well guess what it was: that utter confidence in his capabilities, the never-say-die attitude that had both caused and solved countless problems in their youth.
However, along with that, this Kirk was bitter, hedonistic, very much in the shadow of his deceased father. Never good enough to keep his mother around, foisted off onto unwilling relatives who barely gave him the time of day. He lacked the role models that his Kirk had needed so as not to descend into the anarchic lifestyle that this one had surrendered to. Later, it had been Spock and McCoy who had kept their hot-headed captain grounded, and Spock fervently hoped that his younger self would be that guiding influence once again. And, he could now admit, he had needed his Kirk (and perhaps his McCoy too?) in order to finally bring balance, bring peace to his warring halves.
To have not intervened and brought these two younger versions together would be detrimental to both of them, to Starfleet, and potentially to the entire Federation. Even now, Nero was likely headed towards Earth in order to annihilate Spock's final home. The Federation would survive the loss of Vulcan, and could normally survive the loss of Earth too, but the odds were considerably against enduring with both gone. Only Kirk could come up with a plan insane enough to come close to working, and only Spock could craft that plan into something that was not suicidal.
Apart, they were good; together they were unstoppable. Nero would not know what was surely about to hit him. That was a comforting thought.
A/N: This is actually the first Star Trek piece I've ever written that I've considered fit to be seen (teenage M/S stuff definitely isn't). I kinda hope I did okay.