Okay, first off I get really confused when you say jack-off because then I htink you are a boy because usually males use that term as a looser word than masturbate. So I just wanted to tell you that to describe situations jack-off is not the best word to describe a female masturbating. So I just wanted to let you know that.
Umm you get wet because you get horny. Simple as that because when you are about to have sex or your body gets aroused usually your vagina automatically creates fluid to ease the friction with the penis. It also feels good to the penis too
So taht is why, because you are aroused.