system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If teeth whitening products weaken the enamel, why do dentists recommend them?
gpt
Enamel grows back, like hair. Whitening causes minimal damage. For most people, the enamel grows fast/strong enough that the damage is offset. Elaboration:

We are always damaging our enamel and hair. When you brush/comb you hair you will occasionally pull a few strands out, especially if the brush is untangling a loop/not. Even brushing your teeth you can scratch your enamel. Most whitening tooth pastes are actually designed to scratch the enamel a good deal, so as to polish stains off.

Some products bleach your teeth. Why not just scratch the stains off like whitening toothpaste? Well, your enamel is actually clear once your remove stains. The white color comes from the stuff underneath, called "dentin". Dentin is white for some people, but rather yellow for others. Some people do not like having yellow teeth even though they are perfectly clean. 

So bleaching their teeth will cause their enamel to turn white like they want. This damages enamel worse than scratching, but for most people this will not cause enough damage to do any real harm. Thus dentists recommend it, because it satisfies customers and brings them money.

Of course, some people have weak enamel and should not use whitening products. These people are generally prone to get cavities, and likely have a few already. A good dentist would not recommend whitening to these people. Much like how heavy brushing is not recommend for people with thinning hair.