Why pas consideration is no consideration?

Past consideration is deemed no consideration because, in contract law, consideration must be a present or future benefit or detriment exchanged between parties at the time of the agreement. If a benefit has already been conferred or a detriment already incurred before the contract is formed, it cannot serve as valid consideration for a new promise. This principle ensures that both parties are incentivized to enter into the contract with mutual obligations, rather than one party relying on something that has already occurred. Consequently, contracts based on past consideration may lack the necessary legal enforceability.