The beginning of love is involuntary. It's falling and helpless and head-over-heels. It's a deliriously happy blush that poets and rock stars have spent history and will spend eternity trying to describe to us. But we already know how it feels. We've been there and we desperately want to go back there whether we're single or newly married or divorced or reaching the end of a lifelong partnership. The beginning of love is the inclination to leap without looking. But, sooner or later, we have to look. We have to consider how we will ever go on living if something happens to these people who...