Ishiguro subtly takes on the question of what-might-have-been from a number of different angles ranging from the minutiae of housekeeping details to the tide of international politics (while showing in a kind of Tolstoy-esque way that there's really no way to separate one arena from the other). A splendid book.
Anyway, here's a paragraph I just ran into that captures why I was in a weird mood for a week after first seeing the movie:
But what is the sense in forever speculating what might have happened had such and such a moment turned out differently? One could presumably drive oneself to distraction in this way. In any case, while it is all very well to talk of 'turning points', one can surely only recognize such moments in retrospect. Naturally, when one looks back to such instances today, they may indeed take the appearance of being crucial, precious moments in one's life; but of course, at the time, this was not the impression one had. Rather, it was as though one had available a never-ending number of days, months, years in which to sort out the vagaries of one's relationship with Miss Kenton; an infinite number of further opportunities in which to remedy the effect of this or that misunderstanding. There was surely nothing to indicate at the time that such evidently small incidents would render whole dreams forever irredeemable.