When should we ask the question as negative and when as positive?
"I like ice cream, don't you/do you?"
— I like ice-cream. Do you? This is a neutral question.
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— I like ice-cream. Do you?
This is a neutral question. You don't care if the answer is 'yes' or 'no'. Either way you'll get the information you want.
— I like ice-cream. Don't you?
This question assumes the answer is 'yes'. It's like saying, Of course you like ice-cream too. Everybody likes ice-cream. It would be crazy for you to say you don't like ice-cream.