I do like you - you are emphasizing that you like the person if there is any doubt.

A: I'm not sure that you and I are friends..

B. I think we are, I do like you!

I like you. - you are just saying you like them.

"I like you" is simple and direct and means what it says.

"I do like you" only makes sense as response to a challenge. 

Person A: "I don't think you like me."

Person B: "I do like you."

Like:

Do i like you?

Does she like me?

And

I did not play to football

She does not love you

In present simple we do not need to use "do or does" when we say a statement

its like u mean it