If you favorite something of mine, my way of saying thanks is to look through your work and find something I really enjoy and favoriting it in turn. If I can't find something to my taste, only then will I thank you for a favorite.
If you then thank me for a favorite, I'll then tell you what I enjoyed about your picture. Feedback as to why someone likes something is always nice, and I'd love to know why someone liked a piece of my art.
I don't always get to everyone tho, because I only check DA maybe...once every week and a half.
But don't think I don't appreciate that someone likes something in what I do! <3