Those who have never known the deep intimacy and the intense companionship of happy mutual love have missed the best thing that life has to give.

Russell elevates reciprocal romantic love—marked by both intimacy and companionship—as a central human good rather than a mere private sentiment. The phrasing stresses mutuality (“happy mutual love”) and the double aspect of love: emotional closeness (“deep intimacy”) and shared life (“intense companionship”). Implicitly, he argues that a fully flourishing life is not measured only by achievement, knowledge, or pleasure, but by the capacity to enter a sustaining, shared bond that enlarges one’s experience and mitigates isolation. The claim is also gently admonitory: those who miss such love are not morally at fault, but they are deprived of what Russell regards as life’s richest experiential reward.