Everything has a cost. Everything we have, do, don’t. It may be money, time, effort, energy, thought, space, happiness, health, opportunity, any sort of thing, value. We weigh the value against the cost and roughly determine which things to include in our lives and which to not, to bring them the most overall value, measured personally. This a is sort of learned guess-work, infinite factors boiled down to a manageable set of the largest, most visible, important; past guiding present.