A kind word to the bus driver might be the only one they hear all day.

Some work puts you in front of hundreds of people who all treat you as part of the machinery. Nobody is being cruel, they are busy, and the sum of all that busyness is a whole day with no one speaking to you as a person. One sentence breaks that, and it costs nothing at all.

Say something real to the next person who serves you, drives you or cleans up after you: good morning, thanks, a remark about the weather. Make eye contact while you do it. Say it even when they do not answer.