Why did Spock leave Starfleet, go back to Vulcan, and commit to the Kolinahr?
What was the catalyst?
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It was probably just something he felt he had to do.
Spock always wrestled with his emotions, his human-half, and he wanted to move past those and become more Vulcan. He was unable to devote himself to this path in life because he put his larger goals on hold while serving the five-year mission on the Enterprise. Assuming the same number of years passed between Star Trek TOS and Star Trek TMP, that's an unaccounted-for ten years. I imagine some of that time was spent wrapping up some Starfleet stuff - Spock lecturing at the Academy, writing science papers, or maybe doing some diplomatic work with Romulans or Klingons or whoever - and then the rest was likely spent in study and meditation on Vulcan. Let's say, five or six years preparing for the Kolinhar? A doctoral thesis type deal?
Now, I haven't read the novels or the source books or comics or anything. I've seen the shows and the movies. I know enough about Trek to make those suppositions, but I will bow to anybody who has more "insider" knowledge about the time gap between TOS and TMP and how long it takes a Vulcan to do the Kolinahr.
It could also just be that he wasn't ready. As a half-human, maybe it just was taking a long time for him to purge those pesky emotions to the point where the ritual would work.
But there's one more possibility. He was friends with Kirk and the crew of the Enterprise. Maybe, somewhere inside, he knew he was a complete person being a Vulcan and a human. He needed his emotions, even if they did bother him. So, maybe that's what really slowed him down...
Yup! It's never explained, but it's probably more to do with Spock exploring his Vulcan side than any trauma driving him away from Starfleet and his old life. He could have a billion reasons, he didn't enjoy Starfleet as much after getting his own command, he had stifled guilt feelings about leaving Vulcan, he got sick of human idiocy, and don't we all?
The thing about Spock is that unlike us humans, who have quite a short productive adult life span, and who spend most of that time working to support ourselves and our families, Spock has the luxury of time. He can take spend the odd decade exploring an interest like the Kolinar cult, and go back to research or starfleet or whatever, and he will not have wasted his time.