It's part of the wants and fears system. Sims develop fears from time to time which they can overcome. Sometimes next to wants, you see a special want with a spiky bubble. That's the way to overcome the fear. Read what you have to do and follow the instructions.
Alternatively, you can:
The Brave trait was introduced before the fears were (with the Scared emotion rather). Funnily, it only prevents two fears: Fear of the Dark and Fear of Ghosts (list might be incomplete). Traits that prevent the Fear of Cowplants are Green Fiend, Super Green Thumb, and being a PlantSim. I agree, it's not a good decision to have the Brave trait only prevent a few fears.
