I have compassion for people who are overwhelmed.

The man who was short with you at the counter this morning is three days into something.

That isn't an excuse and you are not required to supply one. It is just the likelier explanation, and it costs less to carry around than the version where he is simply rude and the world is full of rude men.

There is a second effect, less obvious. The generosity you extend outward becomes the only kindness you will have to hand on the day you are the one snapping at somebody in a queue for no reason you could defend.