What should i do if a dentist tells not to fill a cavity ?

Request: If the dentist does not want to fill a cavity, then it is probably not a cavity. It is more than likely staining in the grooves of your teeth from the process of remineralization.

They are Your Teeth: Ask questions of your dentist. You should trust your dentist and the dentist should be able to answer all of your questions to your satisfaction. If the dentist gives you a reasonable and understandable answer as to why they are not filling the cavity, then trust them. If you have any doubts, remember -- they are your teeth so seek out a second opinion.

Ask why not?: There are a number of reasons why a dentist would not want to fill a cavity. For example, if it's a baby tooth that will be lost soon, it it's an non-restorable tooth, if there is significant periodontal disease that jeopardizes the long term prognosis of the tooth, if the tooth has a root fracture, etc. Ask your dentist why not? It should make sense to you. If not, get a second opinion.