Connections
I find it intriguing how closely we are all connected, and how we constantly seek more ways to connect ourselves with others.
Even sharing a birthday creates some sort of connection, which is probably why people are fascinated by knowing people who share a birthday. It turns out that it takes a group of as little as 23 people for at least two of them have the same birthday.
Considering this, it's no surprise that two strangers often find that they have common interests, or have lived in the same state, which in turn creates multiple connections between them.