Why would a speaker of a language tell someone not to learn it?
There are several reasons. Leaving aside personal issues between the to people, there are some more general things:
Language and culture are always interwoven, and some communities prefer to keep these to themselves and not share them with outsiders.
Especially in (post-)colonial contexts, there are communities that do not want the descendants of the colonizers to acquire their language, as it is often one of the few things left to them as token of their identity. More often than not, their languages were forbidden and literally beaten out of them, so speaking it anyhow is important to them in ways outsiders cannot claim to understand.
Two words: cultural appropriation
I am sure I forgot something important here. Dear readers, please feel free to add more…
Collection’s Highlight: Printed Cotton Dress or House Coat
This very busy printed cotton features a rather loud pattern of flowers and paisley. There is an overall plaid pattern created by the alternating verticle green and brown lines and the horizontal paisley bands. The way the material is manipulated is quite impressive- note how well the paisley bands line up. The front of the bodice does not line up as well, and might have been altered at some point.
Dress or House Coat, ca 1860, Cotton, L: 56in. The Fenimore Art Museum, Cooperstown, New York, Gift of Mrs. Fannie D. Spraker, N0064.1973.