To make you understand what this is, I need to bring you back in time to the place where the roots of depersonalization started, in childhood, under - more than likely - psychopath abuse.
Let me remind you to some basic, well known things about emotions. We feel emotions when we experience something important. There is an event in the present moment that makes us feel somehow. From this standpoint, the emotion itself is not important. It can be happiness, anger, frustration, sadness, positive or negative.
But there are a few things we can tell about every single kind of emotion. First one is, that every emotion narrows your attention. The thing that provoked the emotion is the most important to you in that moment. So your awareness focuses on that particular thing. It creates related thoughts in you, generates related emotions, and makes your mind focus on that subject. What does the event mean to you, how it changes your views, what can you do about it, etc..
This is all human. Now, here comes the part where it goes wrong in people who later gets DP. Imagine you are a child, growing up under a psychopath. You experience feelings. But the psychopath doesn't have any, and he is after destroying you. He attacks you every time he can, and you can be sure he never misses a chance. He doesn't have anything in his life, only the psychopath game. He is not distracted by any kind of emotion, because he doesn't have any. And when you experience let's say happiness, the psycho catches you, and starts to bomb you.
He bombs you with negative things, tries to frustrate you, tries to distract you from that positive event and your happiness, or doesn't matter how, but he wants to stimulate and irritate you somehow. To the psycho, it doesn't matter how much or how successfully he can do it, but he does it to the point he can.
Now, if you are a sensitive (caring) person (you are, otherwise you didn't experience chronic DP), you will probably learn to OVERRIDE your emotional experience, and pay attention to the psychopath. Now, that means, you will SUPPRESS the emotion, and the relating thoughts (the ones you SHOULD BE PAYING attention to). The actual emotion will still be there, but you will not experience it fully, because there will be a "defense layer" over it.
If this happens chronically (if you live with a psycho, it definetely happens chronically), over and over again, you will learn that you CAN NOT AFFORD to dwell into your arising emotions, because "life doesn't stop", only if you experience a deep emotion. Because the psycho just comes and comes and comes and comes and talks and talks and asks and shouts and screams like a terminator, in those moments, where the only thing you should be doing is to feel that feeling.
This makes you implicitly learn, that arising emotions (especially positive ones) are a bad thing, because your awareness level will decrease, and you may be attacked and hurt. This is why these people often avoid being happy, because being happy lowers awareness, and when you get hurt while being happy, that hurts the most.
This creates the fog that is a huge huge problem in DPd people. But you all need to understand, that it is a HABIT, and not some kind of WOUND or DAMAGE. It is something "on top" of you, not something that is "missing" from you.