Why do teeth hurt after whitening?

It is believed that the teeth could hurt after whitening because the bleaching agent hydrogen peroxide penetrates the nerve tissue of the tooth and this causes dehydration of the teeth. In addition most professional whitening processes use activating light that causes sensitivity due to prolonged exposure.
Brushing your teeth with a soft bristle tooth brush with desensitizing gel like Sensodyne toothpaste that has a high fluoride content can help get over acute sensitivity. Avoiding both hot and cold stimuli could also help.