What determines who becomes and stays friends? Many factors are linked to friendship, including physical proximity and interpersonal similarities. Recent work has leveraged neuroimaging to detect similarities among friends by capturing how people process the world around them. However, given the cross-sectional nature of past research, it is unknown if neural similarity precedes friendship or only emerges among friends following social connection. Here we show that similarities in neural responses to movie clips—acquired before participants met one another—predicted proximity in a friendship network eight months later (that is, participants with similar responses were more likely to be friends rather than several degrees of separation apart). We also examined changes in distances between participants in their shared social network, which resulted from the formation, persistence and dissolution of friendships, between two months and eight months after they met each other. Compared with people who drifted further apart, people who grew closer over this six-month period had been more neurally similar as strangers. In addition, analyses controlling for sociodemographic similarities showed that whereas these similarities appeared to drive the differences in pre-existing neural similarities between friends and dyads of a social distance of 3, they did not account for the more extensive links between pre-existing neural similarities and the tendency for people to grow closer together, rather than drift farther apart, over time. Thus, whereas some friendships may initially form due to circumstance and dissolve over time, later-emerging and longer-lasting friendships may be rooted in deeper interpersonal compatibilities that are indexed by pre-existing neural similarities. The localization of these results suggests that pre-existing similarities in how people interpret, attend to and emotionally respond to their surroundings are precursors of future friendship and increased social closeness.