system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does the enamel on our teeth wear away and why can't we grow enamel back on out teeth?
gpt
The enamel in your teeth is made from mainly Calcium and Phosphate, and is complexed together in the form of a mineral called hydroxyapatite.

The mineral is very sturdy, making enamel one of the hardest substances in the body, but it's vulnerable to demineralisation when it's exposed to acids in the mouth, usually made as a byproduct of bacteria breaking down sugar without oxygen.

In the presence of hydrogen ions from the acid that bacteria generate, the calcium and phosphate ions become more mobile, and can be washed off or lost. This is what causes tooth decay or caries.

The reason we can't grow enamel back is that the specialised cells that put the enamel on it in the first place don't exist anymore.

Look at it this way, teeth get their enamel while they're still underground and surrounded by blood and nutrition all around them. Once it's exposed to the outside, there are no blood vessels or supplies for any cells to come and repair it. If there were you'd notice them moving around, and even then, those cells would have a really hard time getting from place to place because of the bacteria, saliva and food that's inbetween.

TL;DR
Enamel is dissolved by acids that bacteria produce from left over food and sugar in the mouth.
It can't be regrown because the specialised cells that put it there in the first place don't exist anymore.