Captain Kirk is a man of action, a bona fide space cowboy who famously made his career pushing the limits of the unknown. He is almost literally in love with his starship, the Enterprise. He has a notorious dislike for regulation and the orders of his superiors.
Everything about his character and history suggest that the ideal place for him to be is on the bridge of a starship, serving as Captain, deep out in uncharted space, where he has maximum autonomy and can best apply his skills.
We also know that there are several characters in Star Trek with similar situations who have declined promotions in order to maintain their positions as Captains and explorers. Most notably, Captain Jean-Luc Picard, as explained in the question which inspired this one.
In Kirk's own words, to Picard:
Don't let them promote you. Don't let them transfer you. Don't let them do anything that takes you off the bridge of that ship, because while you're there, you can make a difference.
However, confoundingly, Kirk himself was promoted from Captain to Rear Admiral in between the events of The Original Series and Star Trek: The Motion Picture.
The question is: why?
Why was Kirk promoted to Rear Admiral given his legendary track record as an explorer, desire for the Enterprise, and dislike for authority?
I think we can all agree in retrospect that it was a terrible decision. He's clearly miserable without the Enterprise and takes her back at the earliest opportunity, then proceeds to waste away until the events of Wrath of Khan, and then is promptly demoted for insubordination.