Can I say "why not" instead of "sure"?
"Why not" is a bit of a rhetorical question and is often used when someone doesn't care too much about the topic at hand. It gives off a feeling similar to "that's fine" and thus has a slightly less positive or enthusiastic feeling than "sure" does. Ex: A: Can I borrow your book? B: Sure! Ex: A: Do you want to watch a movie together? B: Why not?
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