I believe our body system works in the way of our behavior. When we are happy, it helps to keep us healthy. But when we are in despair or angry, it will be harmful as the body reacts to such actions. It is always good to think of the good side of life.
Ask yourself this - will it do you any good to be sad or unhappy as compare being happy and satisfied? When you are sad, it creates a sort of harm to your body system unknowingly whereas when you are happy, every part of your systems are working well.
Be happy even though the situation you are facing may not be so. Sadness does not solve your problem but gives you additional problems instead.