Is it “best friend behavior” or something else, like kindness and gratitude? Our expectations of a friend, even a BFF, are very different than what we (generally) expect from a romantic partner, especially if we’re going to have — or already have — children with that partner. We don’t have to work too hard at maintaining friendship; get together as often as we can, talk, text, share. Do you really think that’s enough to sustain a romantic partnership?

From the research I did for my book, here’s what makes for a happy partnership, married or not — matched expectations. When you and your romantic partner have shared expectations, no matter what they are, you’ll basically have a happy, healthy and satisfying partnership. Friends don’t need that.