Captain Willard Decker has a wild couple of days over the course of this film. He goes from being Captain of the Enterprise to being demoted, to rekindling things with an old flame, to becoming the progenitor of a new race of machine-human hybrids.
In comparison, our last couple of days have seen us experiment with various Eggo Waffle combinations, coming to the grand conclusion that the ultimate EggoWich consists of three waffles, peanut butter, banana, honey, and jam.
Who has a fuller life? You be the judge.
Before all of Decker's impressive life-changing activities go down, however, Decker's furious when he learns that Kirk has retaken command of the Enterprise. Even putting aside his pride at not being chosen for such an illustrious position, Decker sincerely believes he can do a better job than a retired admiral, no matter how famous.
Check out this fire:
DECKER: May I ask why?
KIRK: My experience, five years out there dealing with unknowns like this. My familiarity with the Enterprise, this crew.
DECKER: Admiral, this is an almost totally new Enterprise. You don't know her a tenth as well as I do.
KIRK: That's why you're staying aboard. I'm sorry—
DECKER: No, Admiral. I don't think you are, not one damn bit. I remember when you recommended me for this command. You told me how envious you were, and how much you hoped you'd find a way to get a starship command again. Well, it looks like you found a way.
Thankfully, Decker and Kirk's relationship improves after this. They don't always see eye-to-eye—in fact, they're rarely in agreement—but they manage to get over their macho tension. Once everything's said and done, Kirk has a deeper respect for his newly minted executive officer.
But this is small change compared to the Decker-Ilia situation. Decker and Ilia used to have a relationship, and things don't seem to have ended smoothly.
Check it out:
DECKER: I'm sorry.
ILIA: That you left Delta Four? Or that you didn't even say goodbye?
DECKER: If I had seen you again, would you have been able to say it?
So you can probably understand Decker's dismay when Ilia's seemingly disintegrated by V'Ger. You should also be able to understand his confusion (and maybe terror) when she's reanimated as an emotionless android.
Although Decker tries to rekindle Ilia's memories, hoping that the woman he loves is still inside, he ultimately fails.
Ménage A Trois
That is, until he meets V'Ger. After learning V'Ger's origin story and realizing that humans are its Creator, Decker sees an opportunity to reunite with Ilia in a small way by joining her inside the machine. He voluntarily merges with V'Ger after he and the Android Formerly Known as Ilia evaporate into a stream of blue energy.
There are of course a few different ways to read this. But our favorite is that Decker did manage to rekindle Ilia in some small way, and that their love became a part of V'Ger. That might explain why V'Ger is so insistent that Decker be the one who does the merging—it's Ilia's last attempt to merge in a loving embrace with her former snuggle bunny.