Just wondering if there's any difference. Maybe there is... maybe not. However, I do realize that "I like you" has more of a statement-like like kinda feel to it whereas inserting the word "do" in it makes it feel more like, "I like you BUT...".
That's just what I think about this and I'm curious what others think!
I personally interpret "I do like you" as more reassuring, as if you'd just told the person "you don't like me" and they respond with "I do like you". If they don't actually add the "but..." then I think it's kind of silly to assume that they're being insincere or something.
It's like saying "I do like you" is when you are trying to reassure that person that you truly like them. "I like you" is something you say when you a professing your feelings for them, without them asking.