If utilitarianism wasn't practically difficult enough, human perception is adaptable. We don't perceive stimuli in absolute terms, but rather relative to whatever we've normalized. Thus the experience of suffering actually lets us experience pleasure more readily (the reverse is also true).

Additionally, due to the quirks of memory, adding some pain to a pleasurable experience at the right time will paradoxically cause people to remember it as being more pleasurable. This is completely at odds with a simple moral calculus.

It would be interesting to set a science fiction story in a world where an accurate moral calculus had been developed.