Have I got a deal for you! It's called marriage.
In Ancient Greece, businessmen traditionally consummated the signing of contracts by pouring wine on the altar of the gods. The word for this ritual was spendo, meaning, "to pour out a libation," which, because of its association with business, would come to mean, "to make an agreement." In Latin, it begat spondere, spons-, "to speak or pledge," which later proved of assistance in helping a couple espouse their love for one another.
This in turn led to him becoming a sponsus, "one who promised something" — making him into a "betrothed man," and her into a sponsa, a "betrothed woman." Because of the promises made to one another, they evolved into one another's spouse.
A source of joy initially, it was these same promises that caused them to become occasionally despondent when — as everyone is prone to do — they moved de, "away from" their pledges.
A word from our sponsor: Long before it meant, "to pledge," to wed also meant "to gamble" All bets were off, however, when someone weddian another, "for fairer, for fouler."
Today, the wedding is still a gamble, but no longer explicitly so — the risk lurking surreptitiously behind the promises — deal or no deal.