The narrator explains the evolutionary history of eyes: they began as simple light-sensitive cells, and eventually evolved into what we now know as eyes. Studies have shown that when people are attracted to each other, their pupils dilate at the sight of their love interest. Possibly, being able to stare into another's eyes is a display of trust, though it could also be a simple search for connection.
The novel makes several mentions of glances and expressions between characters that clearly communicate something. This adds credence to the idea that eyes exist not just to see, but also to connect with others. In this way, simply looking into a stranger's eyes creates a connection, adding more evidence to the assertion that all people are connected.