Physical pain is a common motif throughout Fight Club. The members of Fight Club and Tyler all revel in what most people try to avoid. They fight until they have serious injuries that require them to go to the hospital. There's also a chemical burn that each member gets. Getting the chemical burn is an extremely painful process and is almost like a rite of passage into the club. What is the significance/meaning of pain in the movie Fight Club?
One thing about physical pain is that unambiguous with a clear relationship between cause and effect.
While emotional pain can be every bit as intense the cause may be a bit more complex and can be related to attitude and context. Indeed you may not fully understand the root cause of emotional pain.
This may also be related to having control over ones life and emotional state. In fight club the pain is the result of a voluntary decision and something you choose to take on, similarly the 'fight' aspect means that you are in a position to dish out pain directly and immediately to the person who is casuing it to you.
This film is often interpreted as a critique of modern life, in which an individual might suffer pain, stress and anxiety from sources which are both ambiguous and remote. For example if you are worried about money there is often no one single root cause that you can deal with....maybe you are bad with money, maybe you don't have a good enough job maybe it's 'the system'.
But of somebody is punching you the source of the pain is clear and it is equally clear what you need to do about it. this is something which humans arer psychologically very well equipped to deal with. You punch back, submit or run away...simple.
Equally if you are fighting in a system of rules where you choose to fight and you fight just for the sake of it then once the fight of fished that's it, you've survived and you move on with the satisfaction that you have achieved something.
Fight Club is a very complex film that has a lot of references and interpretations, so the physical pain may have various meanings in different scenes or contexts. That said my opinion is the physical pain means the only thing that is real for the main character, therefore is the only thing of value in his meaningless and materialist life. When he feels pain he stops evading and focuses in the only thing that is real.