It is considered normal healthy human behavior for a person to have feelings of sympathy toward those around them. The lack or disregard of such feelings is considered a symptom of antisocial personality disorder. People who suffer from this disorder often inflict harm on those around them, manipulating them or using them egoistically to one's own advantage.
When discussing utilitarianism, many critics make the assumption that all parties involved are people with antisocial personality disorder. Never in their theoretical ethical calculus do they consider the negative utility of "feeling bad about hurting people." The fact is, all healthy people should feel bad about hurting people. It is also in our society's best utilitarian interest to maximize this sympathy between its citizens in order to maximize the amount of happiness felt by everyone. If we're all looking out for each others feelings, we're likely to work together toward greater mutual happiness, and thus the greater good is served.
It is also in society's best interest to (humanely) limit those suffering from antisocial personality disorder from hurting those around them, whether it be imposing persuasive consequences for such actions in the form of fines or other inconveniences, or in limiting their contact with others by way of incarceration.
Any rational ethical system based on the idea that people are all sociopaths is likely to appear fairly bleak. The criticism of utilitarianism from people holding to such assumptions is unfair. A rational ethical system need only operate with the assumption of mental health, with provisions for dealing with the occasional aberration.