Why is it so unhealthy to keep emotions to oneself?

People say that if you keep your emotions bottled up inside, it is unhealthy... why? i rarely tell people how i feel. when i get mad i feel like hitting or breaking things, but instead i take a deep breath and suppress it. when i get down, i keep it to myself and just get over it. if i say anything about how i feel its when im happier than normal. i just dont understand why people say its unhealthy to suppress emotions.