Editing other people's stuff is something I've come to really enjoy, and no one is more surprised than I am to hear me say that. For the longest time I couldn't do it. In college workshops I was the person who said, "Yeah, it was great! I really liked it, especially that part with the chicken," if I said anything at all.
Maybe it's about developing empathy, or maybe it's just practice, or maybe it has to do with becoming confident in one's own vision. You have to be open enough to the writer to get what he's trying to do, while keeping enough distance to see where he fails to do it. It's much easier when you know a person well (when you're married to him) to have a sense of he's aiming for, but when you live with a person, you also have to know how to tell the bad news gently. No one wants to wake up next to a cranky-face.
You have to know how much to push your vision for change, and how much to hold back. Don't overwhelm the writer with your fantastic ideas for improvement, and don't lie about liking something when you don't, but don't all-out trash it, either, even if you hate it. It's like a freaking square dance!
I have two big fears before sitting down with the spouse's new book: 1)What if it's so good I die of envy? and 2)What do I tell him if it's awful?
Time to pour a stiff drink.