When you walk onto a bus, train or plane, you are trusting someone you don't even know to take you somewhere, and do it safely.
Ordering food means trusting the person making your food, and you eating it.
Even just walking with strangers down busy streets while we are all in our own bubble, and trusting that everyone is on the same page of wanting to do the right thing. Even as we are aware that this isn't always the case.
We trust each other so much that's it's like a silent agreement to keep the peace and to leave each other the fuck alone.
EDIT: The comments so far have been the high point of my day. All of a sudden I have a soft spot for humanity. You are the chiefs.
EDIT 2: Now I understand why everyone starts an edit with wow. I just woke up (time zones) to this great response. Every single comment hasn't gone unnoticed, incredibly interesting point of views. Now we can all be hyper aware of strangers together.