Why do some people have more/less cavities?

I had my first ever dental cavity fixed today (I'm 22) and my sister has had a dozen of these procedures including a root canal and she's more fastidious when it comes to cleaning her teeth. You know flosses twice a day and the sort. I was wondering what sort of thing makes a person more prone to get cavities? As we're closely related I thought our basic dental 'setup' would be pretty much the same. Just being curious here.

Why do some people have more/less cavities?
I believe genetics is only part of it. Hygiene for sure, the dentist you use, your age, etc. I had fluoride tablets as a small child %26amp; my mom thinks that helped me prevent cavities as I didn't have one until I too was in my 20's. My kids have sealants (expensive but worth it) %26amp; because of them they have no cavities. Some dentists are very conservative %26amp; wait until it's a legitimate cavity to act. Others, often at a fault, think if it looks like one day it might be a cavity, let's go ahead and fill it. One dentist I went to as a young kid said I had like 5 or 6 cavities. I didn't. He just wanted $. Since most people don't run out %26amp; get 2nd opinions on cavities I'm sure there are a lot of teeth getting fillings that don't need them!
Reply:I've heard that some people have a thinner tooth enamel, so no matter how well they take care of their teeth they will always be prone to dental work. Good on you for only having your first cavity fixed at 22! I got my first cavities at 27!
Reply:I believe it mostly has to do with genetics and hygiene. My mother had to get dentures when she was 38 years old, my oldest daughter has had numerous cavities and has to have fluoride treatments every six months due to thin and porous enamel, she brushes and flosses everyday and uses Crest enamel strengthening mouth wash at night before going to bed. My youngest daughter rarely brushes her teeth, and has never flossed her teeth and she has also never had a cavity.