It hurts if you are unlucky enough to get salt in a wound. Namely, salt stimulates the pain nerves.
Applying salt to a wound causes pain, because salt has a stimulating effect on sensory nerves, which respond to stimuli to immediately alert the brain to what is causing the pain.
The sensory nerves respond to pressure and large temperature changes along with chemical changes.
Salt activates the nerves and makes them more sensitive than otherwise, and the pain in the wound will be greater. Chili has the same property and both substances are used in scientific experiments to cause intense pain, for example in muscles.
In addition to this, salt absorbs fluid from the cells and thus induces a major change in their biochemical environment. This can cause many types of discomfort.
