I'm not sure about how/why she died, but I have seen a few hypotheses on the forums about the boon and curse possibly being the same thing. He might have asked the Nightwatcher for a way to overcome his sadness of his wife's death, and She then made him forget her entirely. He doesn't miss her because he doesn't know her (boon), but he has whole parts of his memory, those of her, gone forever (curse).
Some people find this a little too poetic of a curse/boon combo, since it's stated that the boons and curses do not have to, and a lot of times don't, correlate to each other.