What makes Polonius think that Hamlet "Fell into a sadness, then into a fast,/Thence to a watch, thence into a weakness,/Thence to a lightness, and by this declension,/Into a madness wherein now he raves,?"
Polonius believes that the reason of Hamlet's affliction and bad behavior is Ophelia. He had forbidden Hamlet to see his daughter and Hamlet led him to believe he was in pain because of it.
