It always seemed to me that Kirk accepted the promotion to admiral because it was forced on him by Starfleet. He may not have wanted to leave the Enterprise
, but the pre-TWOK Kirk was more of a team player with Starfleet than not, IMO. But hindsight is always 20-20 and Kirk learned soon enough that he had made a big mistake (something that both Spock and McCoy pointed out). It actually adds dimension to Kirk that he wasn't infallible.
And I do think Spock's reason for leaving Starfleet was as simple as his desire to become a true Vulcan (or at the very least, rid himself of his inner conflict with his Human half). In real life, people have left jobs for totally personal reasons too.
But we're also forgetting that McCoy
left Starfleet too. His rationale for leaving--and going "native"--might have been due to being fed up with Starfleet life after TOS and wanting to live like a regular person. In the TMP novelization, it was implied that McCoy resigned due to resentment over Starfleet politics, namely the removal of Kirk from starship command.