While the rest of the crew's promotions felt like natural steps, it always seemed so sad to me that Spock left in an attempt to rid himself of his human half, like he considered his time on the Enterprise a failure.
I know about the Lost Years books, but let's pretend, they are not exist (it's not a canon and were written after TMP, anyway).
Some of The Lost Years
is based on backstory from Gene Roddenberry's novelization of Star Trek: The Motion Picture
, which was written concurrent to the film's production. It's a fascinating read, to see how the man himself imagined his characters and universe.