KIRK: What is it? You look like you've just seen a ghost.
SPOCK: Perhaps I have, Captain. Perhaps I have.
Although it takes some time for the truth behind Spock's relationship to Sybok to come out, our favorite half-Vulcan is immediately shaken by the sudden appearance of a "ghost" from his past.
SPOCK: He reminds me of someone I knew in my youth.
MCCOY: Why, Spock, I didn't know you had one.
SPOCK: I do not often think of the past.
Why do you think Spock doesn't "often think of the past"? We have an idea: it bums him out. Though it's easy to stereotype Spock as cold and unemotional, he clearly has a mess of conflicted feelings about years gone by.
SYBOK: Spock, it's me. It's Sybok. After all these years, you've finally caught up with me. Don't you have anything to say to me?
SPOCK: You are under arrest for 17 violations of the Neutral Zone Treaty.
Not exactly a warm, brotherly reunion, eh? From our perspective, this further emphasizes Spock's disconnection with his past. Sure, we don't expect there to be hugs and high fives after Sybok just took over a planet by force. That'd be a bridge too far. Despite that, it's weird that Spock acts like he doesn't even know his half-bro.
[McCoy sees his father suffering from a grave illness and agrees to perform assisted suicide to release him from his pain.]
This is the deep, dark memory Sybok unearths from McCoy's mind during their psychic therapy sesh. It's pretty intense. No matter which decision he opted for, it's all but guaranteed that he would look back at this time with pain.
SYBOK: That wasn't the worst of it, was it?
SYBOK: Was it? Share it.
MCCOY: Not long after, they found a cure. A goddamn cure.
Now that's just cruel, dudes who wrote Star Trek. To make McCoy's emotional pain even worse, he's left forever wondering whether his old man would still be alive if he had just stood his ground and refused to end his life. How are you supposed to get over something like that?
SYBOK: Release this pain. Release it. This pain has poisoned your soul for a long time.
It's hard to argue with this statement. We may disagree with his methods, but Sybok is right about the way that the traumas of our past affect us deeply in the present.
[In a hallucination, Sarek sees his newborn son Spock for the first time.]
SAREK: So human.
Here's Spock's whopper of a memory, though it's hard to imagine him actually recalling the moment of his birth. Regardless, the key takeaway here is that Spock's personal trauma is his shame over his half-human heritage.
MCCOY: Jim, try to be open about this.
KIRK: About what? That I've made the wrong choices in my life? [...] I know what my weaknesses are. I don't need Sybok to take me on a tour of them.
As for Kirk, he has no interest in a personalized VR walkthrough of his worst memories. He can visit them whenever he wants, thank you very much. While you might attribute this refusal to mere macho posturing on Kirk's part, the truth is that Kirk—who has no shortage of past personal tragedies—has no interest in going backward. He only goes forward.
MCCOY: I was wrong. This "con man" took away my pain.
KIRK: Dammit, Bones, you're a doctor. You know that pain and guilt can't be taken away with the wave of a magic wand.
We think it's hilarious to imagine a doctor having any sort of special insight into "pain and guilt." "By God, Jim," McCoy might say, "his guilt readings are off the chart!" Jokes aside, Kirk is completely correct here: there are no easy solutions when it comes to matters of the mind or heart.
SPOCK: I am not the outcast boy you left behind those many years ago. Since that time, I found myself and my place. I know who I am.
Although Spock doesn't like thinking about his past, it's clear that he's made peace with it on some level. How do you think he managed to do that? We're asking for a friend. If we were in a guessing mood (and we usually are), we'd say that the camaraderie he feels in the present, aboard Enterprise, is enough to outweigh the traumas of his past.