Sadness sometimes a normal behaviour of a person just only focused on one thing that you make happy instead think about your past when we have with your friends been together going out then thats the only way we could smile.
The space inside your Self is sacred. Let no intruder enter. It is saved only for you and God. The reason we have trouble letting go is because we take things in and make them a part of our Self. When we feel sadness, we say, “I am sad.” The sadness becomes a part of us. When we have wealth, we say, “I am wealthy.” When we feel anger or pain, we say, “I am angry” or “I am in pain.” We allow these ...things to define us. And control us. When we love a person, we make that person a part of us. Beware of making another person a part of your Self. Love, give, interact—but never absorb. Keep your Self intact. Do not add to yourself by taking from another person. Nor subtract from yourself to give to another person.
Therefore, the secret to letting go is the realization that the object of attachment (sadness, fear, wealth, people) was never a part of you. You may have held it close, but it was always outside of *you*. Therefore letting go of that object of attachment will not break you because you existed as a whole before the object and will exist as a whole after the object.