Why is this the answer?

The answer is temper because this is a very clever riddle about how your feelings and moods can spread to other people like an invisible wave. Think of your temper as the way you control your feelings, especially when you get upset, annoyed, or frustrated. The riddle says you "must keep this thing." This means it is very important for you to keep your temper under control and stay calm, even when things do not go your way. When you choose to keep a good, happy temper, you are making a great choice for yourself. The next line in the riddle is a big clue: "Its loss will affect your brothers." What happens when you lose your temper? It means you stop controlling your upset feelings, and you might start yelling, slamming a door, or stomping around. When you lose control of your temper, that bad feeling does not just stay with you. It quickly moves and makes the people around you, like your family and friends, feel stressed, sad, or upset too. The word "brothers" means everyone close to you. The last part of the riddle explains why it is so important to stay calm: "For once yours is lost, it will soon be lost by others." Your angry temper can be very catchy. If you start yelling, another person might get mad right back at you, or they might feel so bad that they also become frustrated and sad. It is like a strong push that makes other people lose their own calm feelings. This clever riddle tells us that temper is a very special thing that, once you let it go, often makes the exact same thing happen to the people all around you. That is why keeping your temper is a wonderful gift you give to yourself and to everyone you care about.