The only persons who regard Hamlet as mad are the king and his attendants, and Polonius is the first the to declare him mad. He thinks so because of Hamlet's behavior towards his daughter. When Ophelia comes to him and tells about Hamlet's odd visit to her, he is convinced that Hamlet has gone mad for love. Although we never know what the real purpose of Hamlet's visit was and whether it was exactly like Ophelia described to her father. Such young women like Ophelia are inclined to be very sensitive to words so it is possible that she exaggerated something or misunderstood Hamlet that time. Nevertheless, Polonius calls such incident as “the very ecstasy of love”. He therefore reports to the king "Your noble son is mad: Mad call I it; for, to define true madness" and to prove it, he shows Hamlet's letter to Ophelia which is full of nice words of his boundless love to Ophelia. Though the king is not convinced of this, so then they come up with the idea to watch their meeting from the hiding.
