system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some people so prone to cavities?

I'd like to know why is it that some people seem to be over the top prone to dental health issues, where as others aren't as unlucky?  

Example, I've taken phenomenal care of my teeth, brushing after every meal, flossing, flouride mouthwash and everything, yet every dentist visit yields another 5 or more cavities.  Then I have friends who skip a day in between brushing altogether, chug soda 24/7 and have perfect, strong teeth.

So, did some of us just get the shit end of the genetics stick, or is there a more obvious culprit?
gpt
There is no such thing as 'soft teeth'. With the exception of some rare disease the composition of human enamel is more or less the same.

There are 3 things that contribute to cavities:

1) Something to cause decay - usually food. Fermentable carbohydrates and acid foods are the main contributors. This is why people that drink a lot of soda can be prone to decay.

2) Oral bacteria. Different people have different compositions of bacteria in their mouths. Not all the bacteria in the mouth causes decay but this is where the family tie-in comes. The types of bacteria in your mouth is typically established by the time you are one and colonized from familial sources. 

3) Buffering ability of saliva. Your saliva is a buffer. Activities that overcome saliva's ability to buffer will cause you to be prone to decay. The worst offender here is snacking. Every time you eat it effectively lowers the pH of your mouth and it can take several hours for it to come back. Teeth begin to decay when oral pH is under 5.5 and so if every 2 hours you have a snack that lowers the pH of your mouth below 5.5 you are going to have a bad time.

It takes a little bit of all of these to cause cavities. But an imbalance in one area can make you more susceptible.