the enamel protects the underlying dentin, which is VERY sensitive
. by the gumline
, the enamel is very thin, and does not extend under the gumline moe than a millimeter, if that. when you brush too hard, for a time, one of two things (or both) can happen--the tissue recedes, and/or the enamel gets worn away. either way, yo're left with a VERY sensitive area on the tooth. there are products/treatments for this, but you need to see a dentist. I'd try something called "super seal" first.