According to John Stuart Mill, why should we choose pleasure or pain as the arbiters of morality?human developed that self esteem if not relate to...

Well, let me start by stating that I don't think we should. However, using Mill's reasoning, there are certain solid reasons we should. First, doing so provides a universal binary scale: you can appraise just two factors, pleasure and pain, rather than duty, religious imperative, etc. This provides a strong universal reference. Second, this unified scale would seem to escape the conflicting claims that so trouble people: what happens when two duties clash? When legal duty and religion clash? This scale dissolves those. Third, it allows for a kind of universal calculus of ethics—a hope of objectivity.