Your teeth expand and contract in reaction to changes in temperature. Hot and cold food and beverages can cause pain or irritation to people with sensitive teeth. Over time, tooth enamel can be worn down, gums may shrink or teeth may develop tiny cracks, exposing the interior of the tooth and irritating nerve endings. Just breathing cold air can be painful for those with extremely sensitive teeth. The two leading causes of sensitivity are enamel loss and gum recession. Brushing too hard with a hard-bristled toothbrush can cause increased root exposure over time. Also eating foods that are highly acidic can erode the enamel increasing sensitivity.
The treatment to reduce sensitivity would be to use a soft-bristled toothbrush or a desensitizing tooth paste. There are many brands out on the market such as sensodyne, crest pro-health sensitive shield, etc. Also fluoride treatments at the dental office may reduce sensitivity.