What do each of the main characters(Claudius,Gertrude, and Polonius) feel to be the cause of Hamlet's madness?
Polonius believes Hamlet is lovesick over Ophelia. Fearing Hamlet only meant to seduce her without marriage since the prince and Ophelia are not social equals, Polonius ordered Ophelia to cut off their relationship. As a result, he believes Hamlet is mad with thwarted desire. It must be said that it is politically advantageous for Polonius to believe this, since Ophelia marrying Hamlet would be a potential "cure" for Hamlet's insanity. Note how when it is shown that Hamlet is not lovesick for Ophelia in Act 3, Scene 1, Polonius still argues that the "origin and commencement of this grief/ Sprung from neglected love" (176-177). It's a last-ditch attempt to join his daughter with a prince.
Gertrude believes Hamlet is simply taking the loss of his father hard. In Act 1, Scene 2, she tries to comfort Hamlet, telling him that losing a father is the natural order of things and that his grief is excessive.
Claudius initially concurs with Gertrude, calling Hamlet's sorrow "unmanly," but as the play progresses, he seems to suspect something else is amiss with his nephew. He clearly views Hamlet's madness as a threat, since the prince's behavior is unpredictable and aggressive. Once Hamlet puts on "The Mousetrap," Claudius realizes Hamlet knows more than he lets on, so he resolves to have Hamlet killed.
In Act 1 scene 2, Gertrude and Claudius fret over what they see as Hamlet's "oddness". Gertrude thinks that it is over is father's death and the “o’erhasty” marriage between herself and Claudius. Claudius doesn't say why he thinks that Hamlet is acting odd, but by his suggetstion that Hamlet stay in Denmark we can assume that he thinks that he fears retribution or is frightened by what Hamlet may do. In act two scene one, Polonius decides that the thing that has been bothering Hamlet is his love for Ophelia, he must be lovesick. He is convinced in this by Hamlet's acting strange towards Ophelia. In Act 2 scene 2, Polonius convinces Gertrude and Claudius that Hamlet's maddness stems from being lovesick over Ophelia. He even stakes his job on it.