system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 : How do popular flouride/tooth sensitivity toothpastes & mouthwashes actually repair and treat tooth enamel when enamel is supposed to be irreparable?
gpt
Tooth enamel doesn't regrow, but it can be remineralized.  That means minerals move into the enamel, making it harder and stronger.

Enamel is made of a mineral crystal called hydroxyapatite.  Inside your mouth there's a natural cycle of mineral loss and remineralization that happens every time you eat.  Acidic foods and acids produced by bacteria growing in your mouth and on your teeth cause tooth enamel to lose minerals.  Acids react chemically with the mineral structure of the enamel, basically dissolving it.  Physical wear, such as from chewing or brushing, can also cause loss of minerals, especially when the inside of your mouth is acidic like it is right after eating.  But once your mouth is clean, the saliva goes to work putting minerals back into the enamel.  Saliva is not acidic, and it contains calcium and other ions needed to remineralize the enamel.  It's basically growing a mineral crystal inside your mouth, much like how rock candy grows in a supersaturated solution of sugar.

Each time you eat you lose minerals from your enamel, and then between meals, if your mouth is clean, the minerals are put back.  But if you eat a lot of starchy, sugary, or acidic foods, or you brush your teeth too roughly, or you don't get your mouth clean enough after eating, the mineral loss will happen faster than the remineralization.  This eventually leads to tooth sensitivity or cavities.  Cavities can't be reversed by remineralization.  When you have a cavity, the enamel is gone from a section of your tooth, so there's nothing to grow the mineral crystal onto.

The food you eat contains starches and sugars, which are sticky.  They stick to your teeth.   Saliva breaks down starches into sugars.  Bacteria eat the sugars and make acids.  This all happens right on the surface of your teeth, where the acids are in a position to cause damage to the enamel.  Bacteria also make a sticky film, a kind of scum that sticks really well to your teeth.  It's called plaque, and you can brush it off.  But if it isn't removed completely, over the course of several days, eventually the older bacteria die, and become mineralized by your saliva, in much the same way that your enamel is.  But this mineralized old bacteria isn't smooth like the surface of your tooth.  It's crusty and porous - the perfect place for new bacteria to live, hiding within that crust where your toothbrush can't reach.  This mineral crust is called tartar or calculus, and it can't easily be brushed off.  It's a little bit too hard for that. It needs to be scraped or polished off.  Once tartar forms on your teeth, it's a vicious cycle: the tartar keeps getting thicker and crustier, all the while harboring more and more bacteria that produce more and more acid that cause more and more mineral loss in your teeth.

That's why it's important to brush and floss soon after every meal.  Brushing and flossing clean away the food and the bacterial plaque before too much acid can be produced and before the plaque has time to be mineralized into tartar.  Just don't brush too hard, because that also causes wear in the enamel, at the exact time when it is most vulnerable (right after eating, when the bacteria are feeding and producing acid).  You want gentle circular motions.  Brushing too hard can also wear down your gums.

Toothpaste contains cleaning agents (detergents) that help to wash away food and bacterial films, and mild abrasives that give a little extra cleaning power for the surfaces of your teeth.  Flossing is important for the sake of your gums.  Food stuck between your teeth and next to your gums is food for bacteria that make acids, and the acids are harmful to gum tissue.  When you lose too much gum tissue, you'll eventually lose teeth.

If you keep up with your flossing and brushing, for most people that will be enough to prevent gum disease and excessive loss of minerals from the tooth enamel.  But some people are a bit lazy or need a bit of help, and that's what fluoride does.  Without getting too technical, when there are fluoride ions in your mouth, it changes the remineralization process.  The fluoride ions act in place of one of the other ions naturally found in saliva, causing a different mineral called fluorapatite to be formed during the remineralization process, instead of the normal enamel mineral, hydroxyapatite.   Fluorapatite is less vulnerable to acids than hydroxyapatite, and so teeth remineralized with fluoride are actually less likely to develop cavities than teeth remineralized without fluoride.  Remineralization also happens more quickly with fluoride than it would with just the ions found naturally in your saliva.  But fluorapatite is not as strong or white as hydroxyapatite, and so you don't want to overdo it with the fluoride.  When your teeth incorporate too much fluoride, it's called fluorosis.

Most people who do a good job brushing and flossing frankly don't need any fluoride.  But too many people aren't good at brushing and flossing or for whatever other reason are susceptible to cavities, and so as a public health measure, fluoride is put into toothpaste, some mouthwashes, and even in the municipal water where some people live.  Fluoride is controversial.  Some people say fluoride does more harm than good, or object to the idea of giving a drug to everyone even when they didn't ask for it.

Toothpaste for tooth sensitivity has nothing to do with remineralization.  Most toothpastes for sensitive teeth contain little bits of material that temporarily fill in some of the holes in your tooth's surface, blocking acids and other irritants from reaching the nerves inside your teeth.  The effect is temporary.  To really get a handle on tooth sensitivity you need to improve your basic oral hygiene, avoid the sugary and acidic foods that contribute to tooth sensitivity, and drink only water between meals.  Of course, see a dentist if there are problems you can't solve on your own at home.