Actually, laughing can lift your depression, either just for a minute or a day. I have felt the numbness you mentioned. That's the illness. You are still you in there. With depression you tend to focus inwardly, and some sensations are lessened. I think the fact that you can laugh spontaneously is a very good sign, even if you don't really "feel" happy. As you progress in your treatment, you will feel more normal. It takes time, so be patient with yourself.