Wow! There are so many different ways to answer that question I don't even know where to begin! It's a good question though!

It could be emotional overload so the body has a built in response to just shut down. A defense mechanism of sorts.
It is something that can be learned, especially as a child, to deal with issues that a child does not have the mental capacity to deal with. A child does not have the ability to "reason" with some things, or to "contemplate" certain things, or to "understand" certain things, or "process" certain things...because they haven't yet developed the maturity to do so and then this is how they continue through life because they were never able to resolve those issues in a healthy way.
Sometimes emotional numbness is a sympton of certain illnesses.
Gosh, I could go on and on, but these are a few of what I believe to be reasons for emotional numbness.
TgrsPurr.