In Alcott's Little Women, why does Jo oppose Meg's marriage?

Jo disapproves of Meg marrying.... not of the groom. Her biggest fear is that Meg's marriage will take her away from the family..... Jo believes that she will lose her sister.

"My dear, don't get angry about it, and I will tell you how it happened. John went with me at Mr. Laurence's request, and was so devoted to poor Father that we couldn't help getting fond of him. He was perfectly open and honorable about Meg, for he told us he loved her, but would earn a comfortable home before he asked her to marry him. He only wanted our leave to love her and work for her, and the right to make her love him if he could. He is a truly excellent young man, and we could not refuse to listen to him, but I will not consent to Meg's engaging herself so young.
Of course not. It would be idiotic! I knew there was mischief brewing. I felt it, and now it's worse than I imagined. I just wish I could marry Meg myself, and keep her safe in the family."

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he'll see those handsome eyes that she talks about, and then it will be all up with her. She's got such a soft heart, it will melt like butter in the sun if anyone looks sentimentally at her. She read the short reports he sent more than she did your letters, and pinched me when I spoke of it, and likes brown eyes, and doesn't think John an ugly name, and she'll go and fall in love, and there's an end of peace and fun, and cozy times together. I see it all! They'll go lovering around the house, and we shall have to dodge. Meg will be absorbed and no good to me any more. Brooke will scratch up a fortune somehow, carry her off, and make a hole in the family, and I shall break my heart, and everything will be abominably uncomfortable.