‘Two bodies attract each other directly as the product of their masses and inversely as the square of their distance.’ It sounds like a rule for simple physical facts, does it not? Yet it is nothing of the sort; it was the poetical way the old ones had of expressing the rule of propinquity which governs the emotion of love. The bodies referred to are human bodies, mass is their capacity for love. Young people have a greater capacity for love than the elderly; when they are thrown together they fall in love, yet when they are separated they soon get over it. 'Out of sight, out of mind.’ It is as simple as that. But you were seeking some deep meaning for it.
— Robert A. Heinlein, Orphans of the Sky