You have to understand the time and age in which Hamlet was created. Women were often considered property of the men who were responsible for them--whether it be husband, father, brother, etc. Now consider Hamlet's motivation for his treatment of Ophelia. Clearly, he and Ophelia had an intense relationship. One might propose that Hamlet was being altruistic when he thrust Ophelia aside--a means of saving her from the insanity on which he was about to embark. On the other hand, one might consider his actions thoughtless and egocentric. Clearly, they had an intimate relationship. This would not have been altogether unseemly for a man of his age, but it would have rendered Ophelia less valued for future relationships approved by her father or Laertes, her brother. Telling her to "get the to a nunnery," would have been ludicrous considering she had not saved herself for the church, and she was not left without guardianship until Hamlet's fateful choices.
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