Source of sadness in your life can be many. It may be because of a disturbed relationship, trauma in family or simply a bad hair day. To feel good grab foods rich in magnesium such as bananas and apricots. You may also consuming folic acid which is present in spinach or maybe say a glass of orange juice.
Include vitamin D in your diet because it will stimulate production og serotonin which is a feel-good-hormone that will keep you happy and relaxed. For that you can turn to mushrooms, low-fat fortified milk and cereals or even take vitamin D supplement; for latter, however, consult a doctor.