Why do paper cuts hurt so much?

Fingertips are among the most densely innervated areas of the body, packed with nociceptors and touch receptors because fine manipulation and tactile discrimination depend on them, and the somatosensory cortex devotes disproportionate area to the hands. So an injury there generates far more signal than the same injury elsewhere. A paper cut is also shallow, staying in the upper layers where the nerve endings are concentrated, without cutting deep enough to cause the substantial bleeding that would otherwise clot and seal it.

The wound's character makes it worse. Paper edges are microscopically serrated rather than smooth, so the cut is ragged and tears tissue rather than slicing it cleanly, and the shallow depth means minimal bleeding to flush and protect it. The wound then stays open and exposed to air, and it gets pulled apart repeatedly because fingers move constantly and you cannot stop using your hands. Paper may also leave chemical residues and fibers in the cut that irritate the exposed nerve endings. Treatment is unglamorous: clean it, and cover it with a bandage or liquid adhesive, because keeping the wound closed and moist both reduces pain and speeds healing, contrary to the old advice about letting wounds air out.