Exactly and from personal experience, I can say that when my Dad died, I'd gone from shock, to laughter at his wake, to bursting into tears, back to upbeat and then just solid depression for a year. Kirk was obviously grieving in the scene following Spock's funeral and while not in pieces, he was still reserved and sad on top of being rejuvenated. Star Trek II had to end on something of a positive note because otherwise it would have been crushing, especially since there were no plans at that time to make another movie.