It's so sad when leaves turn brown! There are a few reasons why this might happen. Sometimes, the plant has been given too much sunlight or water. Or it could be that pests have entered the garden and are eating the leaves. Your leaves turn brown because they're getting too much sunlight. If they get too much water, they'll turn yellow, or the roots rot. For pests, it's worth checking if white spots or other colors are on the brown leaves. If you see those, that could be the cause, but if not, I promise you it's because of too much sunlight.