[RyanGuyardo (OP)]

Nails keep on growing, so do our hair, why don't our teeth do the same?
Imagine when we chew, our teeth get worn out and become shorter, will it actually grow longer?

[RD]

Why do teeth grow only twice?

There is a third crop of gnashers: wisdom teeth.

[SteveFish]

During human evolution two sets of teeth were just the right amount. Now we all are living beyond our design age, so take care of your teeth.

Interestingly, many rodents use their incisors as tools for digging, carving wood, opening nuts, and so on, and they grow continuously. If they can't wear them off quick enough they get too long and the animal dies of starvation. This is why your pet hamster chews his cage at night. Humans have substituted tools for rough conditions and important jobs teeth were used for in our hunter gatherer past.