Why? Early demineralization\decalcification of the enamel of the tooth can sometimes be reversed with remineralizing toothpastes & impeccable oral hygiene. However, the causes of this occurring in the first place usually do not stop & therefore it tends to reoccur with deepening decay! there are no magic treatments. It's best to just get it filled when it is small & easily repaired. No reason not to!
Potential Reversal . Depending on the extent of tooth destruction, various treatments can be used to restore teeth to proper form, function, and aesthetics, but there is no known method to regenerate large amounts of tooth structure. If the cavity is discovered at early onset a high Fluoride toothpaste, high ph rinse along with managing risk factors can be used to remineralization of very small cavity lesions.
Good oral. Hygiene will help toslow down the caries process. However, you do need a filling.
Leave it alone. You can monitor it. Some cavities are self limiting and will just be there. Others are active and need to be treated. Best let a competent dentist help you with this decision.
Cavity in tooth. You do need a filling there is no other way, if you dont fill, you will eventually loose the tooth.