Why is plurasi so painful?

Inflammation, nerves: Pleurisy is caused by inflammation of the lining around your lungs. This lining has many nerve endings that sense pain. Added to inflammation and friction with the lining rubbing/moving with each breath (expansion of the lungs), you are going to feel it.

Nerve supply: The pleural space contains more sensory nerve endings than your lips per square inch and when this space becomes inflamed these nerves are activated. Pleurisy is known to be one of the most painful conditions because of this. Also, because you have to breathe, you can't immobilize the chest wall and reduce the pain. Interestingly, the lung itself doesn't have pain fibers and is pain free.