A common complaint I hear as a dentist is, “Why are my teeth sensitive?” There can be multiple causes for sensitive teeth, not all of which mean your teeth are going to fall out, but some of them require treatment or changes in habit.
The first question I usually ask a patient is whether the sensitivity is isolated to one tooth or multiple teeth. If it is multiple teeth it is usually a generalized condition that could be anything from aggressive brushing leading to gum recession or clenching or grinding your teeth. If it is a single tooth, the aggressive brushing or clenching could be involved but other causes may include a cavity, cracked tooth, injuring to the tooth from biting something too hard or a recent filling among other things. Sometimes people have naturally sensitive teeth and there is no obvious reason for the sensitivity.
The best thing to do is contact your dentist so they can ask some questions, run some tests and take some x-rays to zero in on a cause. It is always better to catch a potential problem early to avoid increased pain and cost. Hopefully, though, it is one of the many simple causes that may just require a change of habit (a more careful brushing technique) or possibly using a different type of toothpaste.