One of the causes of sensitive teeth is wear and tear through brushing too hard, using a hard-bristled brush. This can wear down the enamel and expose the dentine.
To help reduce tooth sensitivity, brush with a soft-bristled brush, taking care around the gums so as not to damage or remove gum tissue. Also, make sure to floss regularly and use interdental brushes. The point is to gently dislodge food debris and plaque. You can also use a toothpaste designed for sensitive teeth.
It is also important to visit a dentist regularly and ask them about correct oral hygiene.