The trouble with the friends of humanity is that they will not feel guilty even if everyone is made thoroughly miserable in accordance with their principle of the greatest happiness of the greatest number. For it seems that their own happiness depends upon the intention of making everyone happy, and such is this dependency, that an odd kind of callousness may arise in the face of even the most terrible consequences of their actions. This callousness—or blindness—was noted by T.S. Eliot:
Half the harm that is done in this world is due to people who want to feel important. They don’t mean to do harm—but the harm does not interest them. Or they do not see it, or they justify it because they are absorbed in the endless struggle to think well of themselves. 
The ancient idea that happiness is a by-product of the struggle for virtue, or incidental to other pursuits, or a personal discipline if it is to be a goal at all (such as the Epicureans and Stoics taught), has been largely replaced by this modern idea that we are obliged to make everyone happy. As David Stove says:
That our primary obligation is to increase human happiness, or decrease misery, is an idea only of the last ten minutes, historically speaking. The human race in general has always supposed that its primary moral obligation lies elsewhere: in being holy, or in being virtuous, or in practicing some specific virtue: loyalty or courage, for example. An obligation to increase the general happiness has occupied little if any place in most moral systems, whether of the learned or of the ignorant. But for the contemporaries of whom I am speaking, anything morally more important than human happiness is simply inconceivable. You can easily tell that this is so, by asking any of them to mention an example of something which they regard as extremely morally bad. You will find that what they give, in every case, is an example which turns essentially on pain. 
It all sounds very admirable. After all, who could be against the alleviation of suffering? Who could not wish that everyone were happy? But, as I have already suggested, the danger lies with boundless ambition coupled with good intention as a sop to conscience, the greatest of all modern conveniences; for therewith the conscionable life is made easy, untroubled by terrible and unintended consequences; indeed one may effortlessly arrogate to oneself good feelings in direct proportion to good intentions, no matter what the consequences.
.....With utopian dreams, it is all too easy to forget that life is a certain way, and cannot be otherwise.
Ultimately it amounts to this: life necessarily involves tension and suffering; consequently if we wish to abolish tension and suffering, life is to be extinguished. And there is nothing illogical in this last reasoning. 
We may be thankful that such dreams are rarely pursued as single-mindedly or consistently as Kolakowski’s logical illustration—thankful, that is, for the messiness of life which the perfectionists and the friends of humanity would like to clean away.
 T. S. Eliot, The Cocktail Party, (London: Faber and Faber, 1974), p. 111, quoted by Thomas Sowell, online at http://www.tsowell.com/quotes.html.
 David Stove, “Why You Should be a Conservative”, On Enlightenment (New Brunswick: Transaction Publishers, 2003), p.173, original emphasis.
 Leszek Kolakowski, “The Death of Utopia Reconsidered”, Modernity on Endless Trial (Chicago: University of Chicago Press, 1990), p.141.