Continuity mistake: Towards the end of the film Nicole Kidman is having her wedding photos taken. The photographer poses her for the shot, but when he actually takes the photo, the background has changed. When he poses her, there are just trees and bushes behind her, but when he takes the photo, the trees and bushes are gone and people are walking around.
Continuity mistake: In the bedroom scene where Joseph discusses the boy's letter with Anna, we see Anna putting on her earrings, interestingly enough, her earrings change in each shot. First we see her putting on her 'left' earring while she is still standing by the bureau, then she sits down on the bed and puts on her 'left' earring again, then she stands up - and in the next shot we see her at the buffet where she is suddenly already wearing her 'right' earring, but miraculously missing her 'left' earring. (00:22:00 - 00:23:25)
Answer: I would say that he was "in love" with her before reading the letters. Once he read them, he kinda became convinced that he was the reincarnated spirit of her husband. He clearly is not, since the original Sean was not really in love with her.
I think it was a grey area between the two...I think in life, we are exposed to and vulnerable to, sin. In our rebirth, sin is erased. We don't remember it. I think we remember love. I interpreted the boy as being the reincarnation of Anna's Sean, but only remembering the love he originally had for her, before it was tainted with lust and temptation.