The human (I think animals also) live inside a shell. The world they create inside their minds is in constant shock with the world they touch, smell, see, taste, listen to and think of. They are ready to live whatever is happening inside their shells, and they are certain that the shell and the physical world are not the same. So they are ready to deny the physical world and protect their shell. and the happenings of the physical world that are not predicted inside their shell become what can be defined as pain.
In my shell I believe that shell is what the budda means by attachment, and what Jesus calls the Temple.