The passenger with no luggage may still be carrying the hardest part of the journey.

What people carry is mostly not in their hands. Someone travelling light may be going to a hospital, or leaving a house for the last time, or holding a piece of news they have not yet said aloud to anyone. None of that shows, and most people take some trouble to make sure it does not. The absence of visible weight is no evidence about how heavy a day is.

Assume, in ordinary encounters, that you are seeing very little of what is going on. A small courtesy costs you almost nothing and occasionally lands on someone for whom it is the best thing that has happened all day.