Past consideration is generally not supported by law because it lacks a fundamental element of a valid contract: the concept of a bargained-for exchange. In contract law, a valid contract is formed when there is mutual assent, which includes an offer, an acceptance, and consideration. Consideration signifies that each party to the contract is giving up something of value in exchange for what they are receiving. It ensures that both parties are making a mutual commitment to the contract.

Past consideration, by its nature, doesn't meet the requirement of a "bargained-for exchange" because the act or value given was not provided in exchange for the promise or obligation contained in the contract. Instead, it represents something that has already been performed or given before the contract is formed.