In Star Trek V: The Final Frontier, Spock's half-brother Sybok exposes the "secret pain" of people he encounters.
Sybok forces McCoy to reveal that he had hastened his own dying father's death, only to learn of a cure shortly after. Spock's secret pain is rooted in Sarek's displeasure with Spock's humanity.
Kirk, however, does not permit his pain to be exposed.
What is Kirk's secret pain?
Any relevant details from script notes and drafts of screenplays, novelizations, and interviews would be most appreciated.
(I tried to ask William Shatner this at the Star Trek 50th Anniversary Celebration, but the queue was just too long....)