I think they equate it to male ejaculation and assume that it means that we are feeling what they feel. They can relate to it.
Similarly, they equate nipple hardness to penile erection and think it means we're aroused. It only means that the nipple is stimulated. It's to help babies nurse and while nipple stimulation can be arousing, it can also be irritating.
But they pinch our nipples and jab our gspots frantically, assuming that they can force us to feel like they do.