The science behind why we synchronize with people

A 2019 study on how strangers, friends, and lovers show different physiological synchrony in different emotional states highlighted several key findings (as quoted from the journal article):

1. Compared to pairs of romantic partners and friends, the copresence of strangers led to greater physiological synchrony across several emotions.
2. …during a passive social activity where no social interaction is required, our findings suggest that physiological synchrony is not necessarily reflective of relationship closeness between dyadic partners.
3. …physiological synchrony may emerge more strongly between strangers compared to dyads who are in existing relationships (i.e., friends and romantic partners). The absence of a preexisting relationship drives humans to synchronise their physiological responses to each other, potentially as a bid to establish social affiliation. 
4. Comparatively, in the presence of a friend or romantic partner with whom one has had prior social experiences with, there is less motivation to be alert to the social cues of the partner and to increase affiliation. 
5. …strangers tend to synchronise their physiological arousal in situations of survival (i.e., when fear is induced) and when group belonging is made salient (i.e., when national pride is induced).