Lots of things.  Decay for sure, but teeth can become sensitive for
obvious and not so obvious reasons.  Sometimes toothbrush abrasion (or
wear from normal function or parafunction--usually grinding or clenching
at night).  Gum recession can expose root surface that can be very
sensitive to temperature or touch.
    Sometimes diet can cause transient sensitivity--esp. diet with high
acid content.  For example, my brother likes green seedless grapes in
season, so his teeth are sensitive every summer.
    If you are due for a checkup, do it to rule out problems.  If there
aren't any obvious causes, desensitizing toothpastes (Sensodyne,
Denquel, Aquafresh Sensitive) work pretty well.  If after a couple of
months there is no improvement, there are agents the dentist can use
that may give additional protection from dentinal hypersensitivity.

Steve