Let man be allowed as a sensible being, to choose natural or sensible good, and even to be under a moral obligation of so doing; but let him likewise be allowed in his other capacities to have other views, and to be under other obligations. A rational being ought to act suitably to the reason and nature of things: a social being ought to promote the good of others: an approbation of these ends is unavoidable, a regard to them implied in the very nature of such beings, which must therefore bring on them the strongest moral obligations. To ask, why a rational being should choose to act according to reason, or why a social being should desire the good of others, is full as absurd, as to ask why a sensible being should choose pleasure rather than pain.
Catharine Trotter Cockburn, "Remarks upon an Essay on Moral Obligation", Philosophical Writings, Sheridan, ed. Broadview (Peterborough, ON: 2006) p. 119