Well?
It depends.
Are your teeth sensitive to cold? Sweet? Pressure
All your teeth? Or just some of them.
Has this been a long term problem or something new?
If it’s been that way for years your teeth may be naturally sensitive (I know this doesn’t explain why).
A cavity often makes teeth sensitive, especially to sweets.
Another problem- if you brush too roughly (or with hard bristles), you can cause your gumlines to recede, exposing more sensitive areas of the teeth. I speak from experience.
Better check with a dentist to find out for sure…
Apparently as you age, your gums recede, exposing parts of your teeth that are not protected by enamel.
Or something like that.
What GuanoLad said is what my dentist told me. I thought for certain that I was infested with cavities (I hadn’t been to the dentist in 7 or so years), but it turned out not to be the case. If it is bothering you, I highly recommend a toothpaste for sensitive teeth. It helped out immensely for me. I buy the Crest™ version because it is the cheapest per ounce and it doesn’t taste awful (unlike the others I tried).
Again, the toothpaste you use can make 'em sensitive. It certainly affected mine. Those that are multi-care or anti-plaque are the harshest. Get one specifically for sensitive teeth.
Maybe if you talked nicer to them they wouldn’t be so sensitive. Remember teeth have fillings too.
Bwa ha ha ha get it? Fillings too?
I know, I know. I’m too punny. 
Maybe they’re still in mourning for the baby teeth.
Losing a child is the worst pain imaginable.
I found the Sensodyne does nothing for my teeth. YMMV.