Mill Prep

It would certainly seem that the act of causing pain would be considered immoral, and that causing happiness would be considered moral. There are critics of this idea however, Kierkegaard said something along the lines of, "The pursuit of Pleasure or Pain? Is not that what motivates a slug?" So while innately acting in a manner that produces pleasure can be viewed as good, and a manner that decreases pain is also good, I don't see a fundamental argument that can be made from a metaphysical perspective (like Kant) in favor of this form of ethics. If one is to argue that the action that has the most positive consequences is best, I would have to ask, best for whom? Aren't consequences nearly always subjective (or at least how one views consquences)?