Perhaps, a painful situation that’s been dragging on for too long, a relationship that’s ended badly, or a friend that’s gone away, a person, a place or a memory.
Perhaps you need to take stock of the situation with regard to some element of your emotional past. The need to be alone in order to contemplate some of the events that have happened.
You need to come to an understanding of what it is, you want out of a certain relationship. You need this space, so that you can center yourself and calm down.
Once you have regained your bearings, you will be fine. Whatever it is, try to get over it, and believe in a happier future, if you want to get that smile back on your face.
There's no reason for you to be unhappy. Life doesn't have to be difficult or complicated. Just be completely yourself.