From Private Property and Communism (1848):
"The direct, natural, and necessary relation of person to person is the relation of man to woman. The relation of man to woman is the most genuine relation of human being to human being. It therefore reveals the extent to which man’s natural behaviour has become human, or the extent to which the human essence in him has become his natural essence. The relationship also reveals the extent to which man’s need has become a human need: the extent to which, therefore, the other person as a person has become for him a need.
"... If you love without evoking love in return – if through the vital expression of yourself as a loving person you fail to become a loved person, then your love is impotent, it is a misfortune."