Sources list two foundational works on how friendships form: Festinger, Schachter & Back’s 1950 MIT Westgate housing study demonstrating the propinquity effect (proximity drives social ties), and sociologist Rebecca G. Adams’s articulation of three conditions for friendship formation, cited in The New York Times article 'Friends of a Certain Age' (2012).
Festinger, Schachter & Back (1950) report the MIT Westgate housing study that established the propinquity effect—physical proximity strongly predicts friendship formation among residents.
Sources:
Festinger, Schachter & Back (1950), Social Pressures in Informal Groups: the MIT Westgate housing study that established the propinquity effect.
Rebecca G. Adams, sociologist at UNC Greensboro, on the three conditions for friendship formation, cited in The New York Times, Friends of a Certain Age (2012).