I was 16 years old the first time it really sank in how much we depend on each other.
It was the summer between my Junior and Senior year of high school. I was on my way to get my senior pictures taken, driving my 1972 Ford Torino, with the souped up engine and the amp driving the bass in the speaker box in the trunk. I was doing something like 45 or 50 miles an hour, which is not a bad thing, except the curve was rated at 15mph.
So, I crashed into the ditch, totaling the car.
Along the way, I totaled a guy’s culvert. Dad thought it would be an excellent idea for me to dig the guy’s culvert out and replace it with one I had bought and paid for, as penance for trashing it in the first place. In a fit of pity, Dad agreed to help me do it.

My location as I dug this culvert out is marked on the map by the red dot. That right angle? That was the curve that did me in.
In any event, I was in a ditch, digging a culvert out that I had destroyed with my car by going too fast. Meanwhile, many people were in cars, hurtling at my head, turning at the last possible moment. If any of them were driving the way I had been driving, I would have been dead.
It was then that it occurred to me for the first time how interconnected we all are. How much of our lives depend on our agreeing to do certain things a certain way. I can drive down the street because we have agreed that you will stop at a stop sign. I can walk on the sidewalk because you have agreed to drive on the pavement. I need not fence my front yard, because you have agreed to stay off of it.
Last Friday, I was driving around the block, testing our car after making a small repair to it, when a guy in a large SUV shot out of a side street directly in front of me, in reverse(!) and I hit him.

To clarify: I am driving along, a guy shoots out of a side street in reverse directly in front of me and I plow into him. It turns out, he had crested a hill, seen a police check point and threw it in reverse and hit the gas. He then went backwards for a while, refusing to stop at the appropriate stop sign and jumped out in front of me, when I hit him.
As you might expect, his erratic behavior drew the attention of the police, who were on the scene almost immediately. They arrested him, of course, and called a tow truck for me. And 23 years later, I am struck once again by how much my life is dependent upon everyone else keeping up their end of the agreement.