Love is that condition in which the happiness of another person is essential to your own.

This line is widely attributed to Robert A. Heinlein and is commonly cited as a succinct definition of love. It is generally traced to his novel *Stranger in a Strange Land* (1961), a countercultural touchstone that explores human intimacy, ethics, and social norms through the perspective of Valentine Michael Smith, a human raised by Martians. Heinlein’s fiction often treats love, loyalty, and chosen obligation as practical commitments rather than purely sentimental states, and the quote fits that broader preoccupation: it frames love as a relational condition in which one person’s well-being becomes bound up with another’s. The line has since circulated heavily in quotation anthologies and popular culture.

Heinlein’s formulation defines love not as a feeling but as an interdependence of flourishing: your own happiness is no longer self-contained because it requires the other person’s happiness. The emphasis on “essential” makes the claim stronger than empathy or affection; it suggests a voluntary reordering of priorities in which another’s good becomes constitutive of your own good. The definition also implies responsibility: if the beloved’s happiness is necessary to yours, then their suffering cannot be morally or emotionally irrelevant. At the same time, it can be read as a warning about the depth of attachment—love creates vulnerability, because your well-being is now partly outside your control.

1) "Love is that condition in which the happiness of another person is essential to your own happiness."