Tooth Fairy don't like it

Last year, my boy lost one baby teeth and he put it under his pillow.He expected Tooth Fairy would visit him and leave some money .....Unfortunately, "tooth fairy"(me) forgot to do that.
Next morning, he came to me and said:" Mom, why tooth fairly didn't show up last night?"
"OH," I really felt sorry at that time, but then when I Looked at his teeth, I got an iden:" You know what, this is a cavity teeth, maybe Tooth Fairy don't like it....so always remember don't eat too much candy,OK?"