What causes Aunt Caroll to select Amy rather than Jo to accompany her to Europe?

From the text:

"I'm afraid it's partly your own fault, dear. When Aunt spoke to me the other day, she regretted your blunt manners and too independent spirit, and here she writes, as if quoting something you had said—'I planned at first to ask Jo, but as 'favors burden her', and she 'hates French', I think I won't venture to invite her. Amy is more docile, will make a good companion for Flo, and receive gratefully any help the trip may give her."