Why did Henry Sidgwick reject Bentham’s theory that people are motivated entirely by pleasure and pain?

Henry Sidgwick rejected the psychological theory held by Jeremy Bentham and John Stuart Mill—that human actions are motivated entirely by the pursuit of pleasure and the avoidance of pain—because he sought to establish utilitarianism on a more rigorous foundation than descriptive psychological claims.1

Rather than relying on the assumption that individuals are naturally driven solely by hedonistic impulses, Sidgwick argued that utilitarianism should be supported through systematic reflection on the morality of "common sense." In his seminal work, The Methods of Ethics, he demonstrated that while many requirements of common sense morality could be justified by utilitarian considerations, the theory itself must ultimately rest on a self-evident axiom of rational benevolence. By rejecting Bentham’s theory of motivation and his specific interpretation of moral terms, Sidgwick aimed to resolve the inconsistencies and vagueness inherent in common sense doctrines, thereby providing a more coherent rational basis for ethical conduct.231