The belongingness hypothesis was proposed by Baumeister and Leary in 1995, who suggested that human beings have an almost universal need to form and maintain at least some degree of interpersonal relationships with other humans. According to the theorists, belongingness is an innate quality with an evolutionary basis, and would have clear survival and reproductive benefits.
Baumeister RF, Leary MR. (1995). “The need to belong: desire for interpersonal attachments as a fundamental human motivation“. Psychol Bull. May, 1995;117(3):497-529.