I greet every person with a silent wish that great joy be in their life.

Said out loud it would be unbearable. Nobody wants a stranger at a bus stop announcing that they hope great joy attends the rest of your life.

Which is precisely why the silence does the work. Nothing is performed, nothing is collected, and the other person carries no obligation to respond - they do not even know it happened. It is about the only generous act with no audience at all, including the recipient. Whatever else it does, it means you looked at a face and thought something about the person behind it, which is more than most of us manage on a commute.