What does Polonius believe is the cause of Hamlet's grief?

In act 2, scene 2, Polonius tells the king and queen, Hamlet's uncle—now his stepfather—and mother that the cause of Hamlet's madness is his love for Ophelia, a love that Polonius would not let her return. Polonius says that he told Ophelia, his daughter, "'Lord Hamlet is a prince, out of thy star: / This must not be'" (2.2.150-151).  In other words, Hamlet is out of her league because he is royalty; therefore, you cannot be with him. Then he instructed her to withhold her presence from Hamlet, to return any love letters he sent her, and to, essentially, completely break off the relationship between them (even though she loves him, and Hamlet, Polonius now admits, seemed to truly love her too). Polonius conjectures that Hamlet, once

repelled . . .  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 the madness wherein now he ravesAnd all we mourn for (2.2.155-160).

Therefore, Polonius believes that when Ophelia began to refuse Hamlet's advances, rejecting his once-welcome overtures of love, he grew sad, then stopped eating, then stopped sleeping, became weakened, became light-headed, and, ultimately, went totally insane. Thus, Hamlet is now crazy, Polonius says.