People like being around someone who notices good things. Complaining is contagious, but so is that.

Mood spreads in either direction. Complaining is easy to join, because it asks nothing of anybody, and a group can keep it running all afternoon on no fuel at all. Pointing out good things is harder work, and it turns out to be just as catching, so people end up wanting to be near whoever does it without being able to say why. You get to choose which weather you bring into a room.

Be the one who names the good thing this week, in a group where nobody else will. Out loud, specific, no speech attached. Notice who follows you into it within a minute.