What is jane austen's purpose in writing in her niece fanny in on making an agreeable marriage

She was talking to her niece Fanny and was tell her that she was surprised that Fanny was in love. For last time she saw her Fanny did tell her about her love but Jane did not think she was in love with him. Though Jane wants to laugh at the thought of Fanny in love she knows it is a serious matter. She goes on to tell her that Fanny should make sure she really loves him and that she has no doubt. Jane herself has no doubt that Fanny could in fact be in love. she says she is not trying to convince Fanny not to be with him but she needs to marry for love. She says the worst thing to do it to marry with no affection. So he may realize that he himself has been deceiving himself and that if when they are together they she acts with a coldness he may realize this. Also that his suffering if any has been there for some time. He may realize he must give fanny up but Jane says this kind of disappointment could kill anyone

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