After Hamlet kills Polonius, Ophelia goes mad and dies. In her madness, she sings songs and speaks in riddles. Many of her songs and riddles are about love. Some people believe that Ophelia's madness is an act, and that she is not really in love with Hamlet. They believe that she is using her madness to protect herself from the pain of her father's death and Hamlet's rejection.
There are a number of reasons why people believe that Ophelia is acting. First, her madness is very sudden. She was fine one day, and then she was suddenly mad. This suggests that her madness is not genuine, but rather an act. Second, Ophelia's madness is very convenient. It allows her to avoid the pain of her father's death and Hamlet's rejection. Third, Ophelia's madness is very symbolic. It represents the madness that can result from love, and the tragedy that can occur when love is unrequited.
Of course, it is also possible that Ophelia is genuinely mad. She may have been truly traumatized by her father's death and Hamlet's rejection. It is also possible that she is simply overwhelmed by the grief of her father's death and the guilt of her own sins. Ultimately, it is up to each individual to decide whether or not Ophelia is acting.