Why do our minds and bodies start to feel unreal and the world starts to feel like there's a glass pane between ourselves and it?

Times where I felt like I'm not really in my body. Feeling like I'm simply a vessel in an empty soul. I'm missing huge chunks of my life, I feel so unreal. I started to look at my own body and started to look around the room wondering if everything is real or not, why do i feel like i'm in another dimension or as if i'm watching myself from far away. 

It's kind of like floating in the air but also still held captive inside our bodies and we can read every little detail that's been going on inside our minds and inside my heart. It feels like I'm paralyzed in the brain but I can move my body. It felt so weird that these are what the human body is experiencing, it's kinda cool in a way but also creepy at the same time.