American contract law will not enforce a promise unless something was given in exchange for it. The doctrine is called consideration, and the Restatement defines it as a bargained-for exchange.
A promise to drive a friend to the airport for nothing is not enforceable, however clearly and unequivocally the promisor expressed a wish to be bound. A promise to pay a taxi driver is.
An American yes, on its own, carries no legal weight at all. The question is always what came back the other way — which is why a yes can be given freely and why its reliability has to be established some other way.