I suggest you then look into why people self harm. Of course, it doesn't seem to make sense, every instinct/habit seems to be to protect the body, but this goes beyond that to identity, being in such numbness and apathy and wanting to just feel something, anything. Also can be a self punishment, feeling like one deserves it.
There's alot more under the surface of things.
Tensing up works to protect vital organs in the body, hence why we tense up when being hit in the stomach. Pretty good reason I'd say...
As for your last point, no idea! I won't claim to know something I don't know. Perhaps it is an instinct that gains more ground and sensitivity the more you learn to navigate in the world of form. You learn to flinch better to better try and protect yourself (key word being try, and an instinctive reflex, it's the best it's got naturally).
Like a baby learning to walk, it can't do it straight off the bat, but it learns, and it's driven to naturally want to walk on two legs.