Why has Ophelia gone mad? How might this be proven?
While it is difficult to determine exactly what causes Ophelia to go mad, there are several contributing factors that may have some connection to her madness.
Ophelia is a generally weak character; she bends to the will of everyone: the king and queen, her brother, her father, and Hamlet. When these people are removed from her, or disapprove of her, she breaks.
When Hamlet mistakenly kills Polonius later in Act III, Ophelia goes mad. Her madness may well be a result of a combination of factors:
1. She loves Hamlet and is not only shunned by him, but treated horribly by him.
2. Her father, whom she also loves, is dead.
3. Her father has been murdered by the man she wants to marry.
4. She is alone in the world now that Leartes is away, her father is dead, and Hamlet has completely rejected her.
5. She may be pregnant. (This possibility is indicated in Act IV Scene 6 when she sings about a maid losing her virginity and later in the same scene when she gives herself rue--which could be used to cause a miscarriage.)
Also, rue, when used incorrectly as an abortifacient, may cause irratic behavior, or "madness"
Check out Enotes page on Hamlet for more information.
There is an interesting web site that analyzes the meanings behind the different flowers Ophelia presents to people during her second mad scene (Act IV Scene 6). I have added a link to that page below.