Hey there smoosher. I’ve been pondering how to answer this one for a while. The reason being I don’t really know.
Scientists and philosophers have been looking for an answer to this particular problem for a long time, and while several theories have been proposed, a lot of people now it emerges as a result of how are brains are organised. We have millions of neurons, that make billions of connections with each other in our brain and from all these connections, there comes the idea of “self”.
Researchers who study the brain can take pictures of it in different states of consciousness, to see which parts are active. It might be possible to find out whether there are specific regions that are active in a “conscious” state, but not active when “unconscious”. The problem is that it doesn’t appear that consciousness can be located to any specific area. It looks like many different parts of the brain are involved.
There was a piece on the BBC news about this earlier this week. If you can see the video, then you get some idea of the different bits of the brain that are active in any given state: