we usually use "do" after someone asks us the question.

"do you love me?"
"yes, I do love you"

but they are both similar, and they can both be used pretty much anytime.

@Dave: It seems have no difference between those twe expressions. Okay, it helped. Thank you :)

"do" in this case is an auxiliary verb which does emphasize the primary verb.

It can also help descibe the time frame or situation

'I love you' - general
'I do love you' - emphasize present
'I did love you' - past
'I dont love you' - negative
"He does love you" - third person

So in english we will us auxiliary verb to better describe a primary verb. It is like an adjective, but not really. I hope you are not confused. ;;;;;;

우리, 최선을 다합시다!

Maybe this is just my feeling, but we use the aux 'do' more often in response to doubt because, as you said, we want to emphasize the fact or the act of doing something. If someone doubts us, we want to assure them that we did in fact do that action.

You didn't do your homework, did you?
I really did it. (I honestly and/or completely did it.)
I DID do it. (emphasizing not how the homework was done but the act of having done it.)

Did you ever love me?
Of course I loved you. (general statement of past love)
I did love you. (emphasizes the act of loving in the past)

You don't love me, do you?
I did love you. (emphasizing the fact that I loved you in the past, but maybe not now)
I do love you. (emphasizing the fact that I love you because you asked and seemed to doubt.)

In many situations, these different ways of emphasizing are interchangeable -- they can mean the same thing. But there is a nuance. Other ways of emphasis ('indeed', -ly adverbs, etc.) emphasize HOW some action, but the aux 'do' emphasizes whether some activity was done or the fact that it was done.

Why should I help you?
You do love, don't you? (The fact that you love me suggests that you have a good reason to help.)

That doesn't make any sense, does it? Well, I did try.

I don't think I have ever heard someone say "I love you indeed", it's a bit strange.

Normally, I hear indeed used like this.

A "It's raining so hard hard outside"

B "Indeed..."


C "This food is so good"

D "Indeed"

But when people say this it is kind of funny because it's a strange thing to say.