It's because of ----
1. Dental caries/
2. Wrong brushing technique with course or abrasive toothpaste /
3. Gum disease called gingival recession causing exposure of cementum the protective layer of tooth root.
So, for further information about it,
Please get in-office consultation with dentist for further clinical examination & treatment accordingly.
Or
Consult online through Practo for further information about it. Let me know if, I can help you to find out best solution for it.
Health Tips
follow proper brushing technique with ultra soft toothbrush & don't chew anything from that side until the treatment is done.