Full Idea
No reason can be given why the general happiness is desirable, except that each person, so far as he believes it to be attainable, desires his own happiness.
Gist of Idea
General happiness is only desirable because individuals desire their own happiness
Source
John Stuart Mill (Utilitarianism [1861], Ch.4)
Book Reference
Mill,John Stuart: 'Utilitarianism (including On Liberty etc)', ed/tr. Warnock,Mary [Fontana 1962], p.288