I usually have that reflection when I'm travelling. It's indeed a depressing feeling. Last time it happened I was in the middle of Rome/Italy with many unknown faces around me, that I was seeing for the first time and - thinking at that time - that I will never see them again.
I've had a melancholic moment traveling in a bus on the narrow roads in Nepal, and seeing some rock faces, just realizing that while the rocks will be there long after I am gone, I will never set my eyes on the same rock face again.
So many years later and I still think about that rock face for some reason.
