Why does our body react to pain/sensation felt by someone else?

For example in the gif below I cannot help but to feel ticklish in my crotch area http://i.imgur.com/WpSBc.gif

And when watching medical procedures on TV i cant help but feel pain the the areas the doctors are operating on the patient.

Finally, men feel pain in our nuts when watching others getting their nuts busted.

Why are we feeling the pain that is not physically related to us?