Why does bleaching cause tooth pain?

Reasons: Teeth are made up of a hard outer layer(enamel) and a softer layer underneath (dentin). Dentin has nerves and when exposed becomes sensitive. Whitening temporarily makes the enamel more permeable and microscopically exposes the dentin. Also the trays used in whitening can put pressure on the teeth which can cause them to become sore.

Not sure: The companies say because teeth get dehydrated during the procedure there is sensitivity. I believe the whitening solution creates a temporary change in the dentinal fluid movement which transmits pain. The sensitivity is temporary.

Sensitivity: Whitening gels can cause sensitivity in some instances, it could be decay or just recession of the gums, see your dentist for an evaluation.

Possibly...: Bleaching is recommended for healthy teeth and teeth that are cavity-free. This has a way of decreasing the potential for decay and damage. However, bleaching can still cause pain. It does so because it removes accumulated stains and debris from the porous enamel tooth structure. As a result that which blocked sensitivity is removed from the bleaching process. To counter this dentists use fluoride.

Nerve irritation: The solution irritated the nerve to the tooth.