Why do some people have extra saliva in their mouth and does that affect cavities?

Differences: Some people just produce more saliva than others and that all can be affected by meds, eating habits, and stimulation. More saliva is actually better to resist cavities. Saliva has buffering abilities that reduce acidity and some antibacterial properties that help keep normal bacteria in check. Decreased saliva can lead to increased decay if hygiene is insufficient. See a dentist if concerned.

Extra saliva : Saliva has various components in it and yes having extra saliva could be good for less cavity's as it helps wash away the sticky food.