Sometimes I think it's a coping mechanism in order not to feel anymore. You are depressed or sad for so long that eventually you automatically 'stop feeling' in order to protect yourself from emotions. But unfortunately you pay a sacrifice because you are no longer able to feel happiness normally either. I think the key is starting to feel comfortable with your emotions again. When you're happy, try to look at it as a normal thing instead of a strange or rare thing. Same thing with when you are sad, or whatever other emotion you might be feeling. Embrace your emotions instead of running away from them and retreating to your emotionless world. You have to admit to yourself that emotions can be good things, not just bad.