. . . As he stands at the transporter after packing his things, something dawns on him and he cries, "No! I can't leave!"
Actually, it's more like:
"NO! . . . I . . . can't . . . LEAVE!!
Shatner's drawn-out delivery drives home the point, I think. There's an inner conflict going on between the spore influence and Kirk's dedication to his command and his ship. And, of course, we know which has to win in the end.