genetics and which bacteria take up residence in her mouth (which is familial also, because you're kissing her, biting off food for her, etc). there are hundreds of strains of bacteria in there and only 2 cause cavities. those 2 have built subdivisions in my mouth for sure. i asked my dentist (who's very research-oriented) if they were considering using probiotics yet, and he said no, but it seems like a good idea to me. there's also xylitol gel which is a prebiotic (food for the bacteria) for the good guys, and that's pretty easy to rub on her gums/teeth, plus it's sweet.