We'll admit it. We're still not over the death of Leonard Nimoy. Even though he lived a full, long, rich life, it still seems like he was taken from us too soon.
And a lot of that has to do with the fact that Spock, Nimoy's alterego, seemed like he would live forever.
But that's not to say he's not a dynamic character—like Kirk, Spock has been going through some serious life changes in the time between the original series and Star Trek: The Motion Picture. He's quit Starfleet to focus on reconnecting with his Vulcan roots, which seems like practically a full-time job.
Digging up Roots
He's been doing this through the Kolinahr ritual, which is a ceremony though which Vulcans abandon their emotions in favor of pure logic. This is a huge deal for their culture: Vulcans had once been a highly emotional and volatile species, but actively abandoned their emotions in order to evolve as a society.
Because of this, achieving Kolinahr is an important rite of passage for any Vulcan. It's like a Vulcan First Communion or Bar Mitzvah.
This is especially important to Spock because he's half-Vulcan and half-human, a blend that makes him a hair more emotional than your average wearer of pointy ears. This also makes him eager to prove his Vulcan heritage—he seems to relate much more to that side of himself. (Fair enough. Humans can be horrible.)
Unfortunately for him, however, that human side isn't going down without a fight, as evidenced by his refusal to complete the ritual due to the messages he's been receiving from the entity in space.
Check out what happens after a Vulcan leader "mind-melds" with Spock, which allows her to read his thoughts:
FEMALE VULCAN: The consciousness calling to you from space, it touches your human blood. You have not achieved Kolinahr.
Talk about irony: Spock started the Kolinahr ritual to get closer to his Vulcan roots, but ended up getting closer to his human half. Ouch. Although he might be disappointed, Spock decides to reunite with the Enterprise in the hopes of learning more about this strange consciousness calling him from outer space.
Down the Rabbit Hole
The one bright side to this is that it reunites him with his former comrades. Although Kirk's concerned about his old friend, he refuses to consider the possibility (suggested by Bones) that Spock may betray them somehow due to his connection with the entity. Kirk knows that he and Spock have been through way too much for that to be a possibility.
Later, Spock comes in direct contact with the entity known as V'Ger after mind-melding with it. In an interesting reversal from when Spock was 100% behind the idea of getting his Kolinahr on, the concept of "pure logic" is now framed in a negative light.
SPOCK: V'Ger has knowledge that spans this universe. And, yet, with all this pure logic, V'Ger is barren, cold—no mystery, no beauty. I should have known.
KIRK: Known? Known what? Spock, what should you have known?
[Spock takes Kirk hand.]
SPOCK: This simple feeling is beyond V'Ger's comprehension. No meaning, no hope, and—Jim—no answers. It's asking questions. "Is this all I am? Is there nothing more?
In many ways, V'Ger is experiencing the exact same struggle between logic and emotion as Spock. The only difference is that Spock is actually able to have feelings, while V'Ger has no choice but to cling to its precious logic. This instills a further appreciation within Spock of his emotional, human side, as well as revealing to him how to interact with V'Ger.
Spock realizes that, like most of us, V'Ger feels like a child with an overwhelming sense of need. (Who hasn't felt like that on a bad day? Or, um, on most days?)
The compassion Spock shows to V'Ger is evidence of his growth over the course of the film. He now realizes that he doesn't have to choose between emotion and logic—he can have both.