I have a lot of extra saliva in my mouth. Why do I have so much in my mouth?

People differ. Some people just salivate more. Stay hydrated.
A good thing. Saliva is a beneficial thing, some people don't produce enough. Without it the teeth would not have any defense against tooth decay and the acids from the foods that we eat and drink. Your system may just need to create more then you think is needed to take care of the environment in your mouth. Of course when there is a signifcant problem it is always good to have it evaluated by your dentist.