Because it is crazy, if you think about it, to decide to commit to another person even though you have no clue who each of you will be in ten, twenty, thirty years’ time. You make a pact to build a life with someone without knowing what that life will look like. Without knowing who will get sick or who will lose their job, whose sex drive will change or whose in-laws might need regular care. This is what we do when we begin a relationship: we commit to something unknowable.