I have this sort of train of thought randomly all of the time. It usually happens in the car. It will be a weekday, and there will still be so much traffic. And I'll wonder what everyone is doing and wonder what everyone's life is like.
I was a deep thinker as a kid too. I think one of my biggest realizations on life was on my eighth birthday. I started crying, because I realized I was the youngest. My parents and siblings would die before me, and I would be all alone one day. And every year that I got older, they did too.