In The Great Gatsby, why doesn't Nick want to leave Gatsby the morning after the accident?
It is in Chapter Eight that you need to look to find the answer to this question. Of course, we are left to infer Nick's motives from his narrative, and yet the central reason why Nick does not want to leave Gatsby is because he recognises what Gatbsy is only just beginning to recognise: that Gatsby has failed in achieving his dominant dream--to gain Daisy. Note what the text says just before Nick tells us how reluctant he was to leave Gatsby:
The track curved and now it was going away from the sun, which, as it sank lower, seemed to spead itself in benediction over the vanishing city where she had drawn her breath. He stretched out his hand desperately as if to snatch only a wisp of air, to save a fragment of the spot that she had made lovely for him. But it was all going by too fast now for his blured eyes and he knew that he had lost that part of it, the freshest and the best, forever.
Nick knows how this dream has consumed Gatsby for so many years, and how everything that he had achieved in terms of his wealth and prestige has been aimed at the one goal of "winning" Daisy, and thus he is immensely concerned about the impact of this destruction of dreams on Gatsby and how he will react. This is why he does not want to leave him alone.
Gatsby has poured out his heart and life story to Nick in chapter 8 of the novel before Nick realizes that there are only twelve minutes until his train, and then deliberately allows himself to miss it. He offers us one explanation as to why he doesn’t want to leave Gatsby, because he “wasn’t worth a decent stroke of work,” but then quickly dismisses it. He does not want to leave Gatsby, really, for personal reasons.
Nick doesn’t tell us what these are exactly, but we can guess. Before he finally tears himself away, he tells Gatsby that he is “worth the whole damn bunch put together.” Nick had never given him a compliment before, so we know that to be driven to say this, Nick is surely feeling concerned about what Gatsby’s current mental state must be. He knows that Gatsby has lost his dream and his friends, the young man he created a “shattered” illusion. It isn’t clear whether or not Nick is worried that Gatsby might harm himself physically or mentally, or that some harm might come to him from another quarter, but it is clear that he is worried about him in general and some part of him, without offering an explicit explanation, doesn’t want to leave him alone to dwell on the despair he must now feel at having lost everything he had dreamed of having.