'Do' and 'does' is used in statements to add emphasis. 1. Sometimes, you can add 'do' simply to make the statement stronger. 'I do love you' means 'I really love you'. This is rather old-fashioned, and sounds slightly unnatural these days. People in romantic films in the 1940s and 1950s made this kind of declaration. 2. It can provide a contrast. For example: I don't like sweet things. I never eat cakes or biscuits or anything like that. But I do like chocolate. 3. It can indicate a contradiction. For example: You don't like Jack, do you? I do like him, actually. He's ok. I hope that helps.

When you use 'do', it is in the viewpoint that you are answering a QUESTION that was imposed by another; or bringing across a point after having said/done something related to the phrase. Do you love me? -- I do love you. You've gotten fatter, but I still DO love you no matter. You tentatively only use DO if you want to use it, because you can always answer with 'I love/hate/like you'. This is my take in general.