night nursing and tooth decay

can night nursing cause tooth decay? I'm still confuse on how it can but the dentist seems to think so. I wish my lactation consultant would have warned me if so. also, seems like tooth decay has been on the rise. wish someone would have told me I could start brushing her teeth with a bit of flouride instead of waiting for her to turn 2. by the time she turned two it was too late to do something about it beside anesthesia and caps. Feel like a failure mom.