because we (as readers) feel we *deserve* this level of romantic intensity at this point in the story -- we've gotten so little of it up to this point. Just as Anne had only a fleeting taste of passion and romance, followed by years of living on memories and months of living on hope, so too we've had to live on Anne's memories (which we never get in any detail) and her hopes. The letter is like all that black-and-white finally turning into Technicolor.