By self-interest Man has become gregarious, but in instinct he has remained to a great extent solitary; hence the need of religion and morality to reinforce self-interest.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“Hence the type of man encouraged by romanticism, especially of the Byronic variety, is violent and anti-social, an anarchic rebel or a conquering tyrant. This outlook makes an appeal for which the reasons lie very deep in human nature and human circumstances. By self-interest Man has become gregarious, but in instinct he has remained to a great extent solitary; hence the need of religion and morality to reinforce self-interest. But the habit of forgoing present satisfactions for the sake of future advantages is irksome, and when passions are roused the prudent restraints of social behaviour become difficult to endure. Those who, at such times, throw them off, acquire a new energy and sense of power from the cessation of inner conflict, and, though they may come to disaster in the end, enjoy meanwhile a sense of godlike exaltation which, though known to the great mystics, can never be experienced by a merely pedestrian virtue.” source