Why does proximity seem to promote relationships between people? because reciprocity is a natural consequence of proximity. because proximity promotes familiarity. because proximity makes it likely that both similarity and complementarity will develop. because proximity necessitates similarity?

The answer is: because proximity promotes familiarity
Humans are most likely to let our guard down if we're near the people that we're familiar with.
Due to the close relation that created by proximity, we're most likely to meet that individual much more often compared to other individual, which make us much more familiar with that individual's behavior and personalities.