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"Don't You" is about a person (the singer) who runs into a former love out of the blue -- someone who she still has feelings for. but her partner has moved on and has a new girlfriend/wife, but the singer still loves the partner (will use he/him pronouns for clarity)
the key to the song is in the title: "don't you." don't you is a shortened version of the phrase "don't you dare," which is said to another person to tell them not to do something, otherwise it will make the speaker angry or upset. ex) don't you dare slam that door!
so in the first half of the chorus, Taylor's character is singing to him: don't you (dare) be friendly and act nicely to me because "you don't know how much I feel I love you still." she still has feelings for him and him talking to her and being happy makes it worse.
but that's not the only use of "don't you" in the song because Taylor does the thing she does so well, which is to take a common phrase and twist it.
in the last two lines of the chorus, she turns "don't you (dare)" into "why don't you?" as in "why don't you love me anymore?" so she goes from an angry demand to a tragic plea for him to love her again.
this happens again in the bridge where she says she'll "go on with my life" and he will say "we're still friends" (the worst) but she doesn't want to pretend to be friends because to her, he's still the one she wants.
so anyways, Taylor's wordplay between "don't you (dare)" and "why don't you" is so effective here in showing the magnitude of the sadness, regret, and hurt of the singer and again shows why she's a master songwriter /end
