Saliva prevents cavities, and lack of it causes tooth decay. People who smoke have drier mouths, which puts them at a higher risk of tooth decay. I had a friend in college who played flute and this would cause her mouth to dry out, which also resulted in cavities.

I used to not care for my teeth at all back in HS, and ended up with one small cavity 20+ years ago. That was the last cavity, and years of fluoride rinse and good hygiene mean my X-rays look better now than they did in my 20s