Why do some people put an extra "DO" in a sentence? [duplicate]

Solution 1:

Question: Do you/Don't you like her?

A: I do like her...

B: I like her.

case A: The word do suggests emphasis here on 'like' or 'her', and it is usually an answer to a question or it is followed by something.

I do like her but some of her habits irritate me

it's like saying, as a matter of fact, I do like her..(the emphasis is on liking her, and he might want to talk more about other qualities now..)

I do like her but I am not ready to pop the question anytime soon

I do like her but her brother gets on my nerves

Case B:

It is pretty straightforward and self-sufficient,stating the fact or giving information that he/she likes her. There is no emphasis. And sounds casual and nice.