People can knock on your mood. They don’t get to move in.

Somebody walks in already irritated and ten minutes later you are irritated too, about nothing of your own. Moods travel, especially from people you are close to, and you usually catch one before you notice it arriving. Feeling it is not weakness. Handing over the rest of your day is the part you have some say in.

When your mood shifts, stop and ask whose it actually is. Often you can trace it straight back to one conversation. Naming it breaks most of the spell, and then you can be decent to the person without taking on their weather.