Ophelia seemed to be desperately in love with Hamlet, whom rejected her. She finds it difficult to understand Hamlet's actions, for he later seeks attention once again, suggesting rather sexual references. It's the knowledge of her fathers death and that Hamlet had treated her so unfairly, is what drives her to her breakdown, for she felt emotion for Hamlet. She continuously sings helpless melodies about death, heartbreak and lost love, broken promises etc. This created a concern in Gertrude and Claudius overall, about Ophelia's mental health.
