Laughing at the same moment is a kind of agreement reached before language. Two people notice the same absurdity in the same half-second, and what the laugh confirms is that you are seeing the room the same way, which is most of what closeness is made of. That is why it feels disproportionate, and why an unshared joke leaves such a particular loneliness.

When you find someone who laughs at what you laugh at, treat it as information rather than a coincidence. It usually points at a friendship worth spending time on, and time is the only thing that turns the moment into something more.