system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are the differences between different types of toothpaste (eg whitening vs enamel strength vs gum repair) and why doesn’t one toothpaste just do everything?
gpt
Dentist here. It's the additives. Pretty much all toothpastes are an abrasive and fluoride. The abrasive is to amplify the efficiency of your brushing and the fluoride to help fix the damage caused by the acid erosion from the bacteria. Under the enamel is the shock absorber of the tooth, dentin. The dentin has a matrix of water tubes originating from the root of nerve and blood supply of the tooth.   
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Everyone has a different thickness of enamel, sensitivity of the dentin, size of the pulp chamber etc.   
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The different toothpastes are made to focus on the individual's marketed advantage. Do you have gum disease? How about a toothpaste with an additive that will kill more fun disease bacteria? Want whiter teeth without paying for whitening? Want that old wives baking soda clean that got grew up knowing worked? Maybe a cool mint before bed because the strong mint wakes you up?   Do cold things bother you? How about a toothpaste that will seal up those dentin tubules to make the tooth less sensitive?   
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As you can see they have made a toothpaste for pretty much every preference. As a dentist, they all work great, except the charcoal. That will ruin your teeth.