If a tooth already hurts, won't covering it make it hurt more?
It is a fair question. It assumes the pain comes from pressure. For most people it does not.
The FDI World Dental Federation names the main cause as open dentinal tubules.
In plain terms: under your enamel sits a softer layer.
It is full of tiny channels that run toward the nerve.
Enamel wears. Gums pull back. Edges chip.
When that happens, those channels are no longer covered.
Then something reaches them. Cold air. Ice water. Sugar. A toothbrush.
Fluid inside the channels moves, and the nerve reads that as a jolt.
Dentists call this the hydrodynamic theory. It is the standard account.