Social touch plays a powerful role in human life, with important physical and mental health benefits in development and adulthood. Touch is central in building the foundations of social interaction, attachment, and cognition, and early, social touch has unique, beneficial neurophysiological and epigenetic effects. The recent discovery of a separate neurophysiological system for affectively laden touch in humans has further kindled scientific interest in the area. Remarkably, however, little is known about what motivates and sustains the human tendency to touch others in a pro-social manner. Given the importance of social touch, we hypothesized that active stroking elicits more sensory pleasure when touching others' skin than when touching one's own skin. In a set of six experiments (total N = 133) we found that healthy participants, mostly tested in pairs to account for any objective differences in skin softness, consistently judged another's skin as feeling softer and smoother than their own skin. We further found that this softness illusion appeared selectively when the touch activated a neurophysiological system for affective touch in the receiver. We conclude that this sensory illusion underlies a novel, bodily mechanism of socio-affective bonding and enhances our motivation to touch others.