When I tell a friend I’m writing about how to have a stress-free marriage, she laughs bitterly. Like many couples, she and her husband battle over money: he’s an avid saver and investor while she prefers to live in the moment and deal with the consequences. I don’t know any couple that doesn’t come into conflict over the division of chores, feels let down by a hastily chosen birthday present (or, worse, no present at all) or seethes when their partner is too exhausted to chat after a long workin