Dear friend, I'm afraid that no matter what your intentions are and how much you're grateful for giving you a fav, leaving "Thanks for fav" comments is SPAMMING. I'm serious. Just take a look at the comments section from the main page of literally any DeviantArt user's account you'll find. Such comments are EVERYWHERE. When you favourite anything, there's a 50% chance that you'll get one.
You may be asking yourself "But why should this be a bad thing?" Well, it's not a bad thing per se. It's nice to know that someone is grateful for faving their artworks. And if you just started using DeviantArt and created an account here, then you'll probably be actually happy for receiving such comments. You will still be happy after 10 such comments. Maybe you'll still be happy after receiving 50 of them.
...But when you receive the very same comment for the 1000th time, and your comments section starts looking like the "Spam" folder on your e-mail account, filled with the very same, copypasted message that you are always notified about and are forced to at least glance at it before removing it... that kinda gets annoying, you know?
Do those messages serve any real purpose? Not really.
Are they useful? Not really.
Do you achieve anything for receiving or writing them? Not really.
Can they be annoying? Yes, they can.
Are you wasting your and other people's time when writing them? Sadly, yes you do.
Are they mixing up with comments that may actually have some purpose? Yes, they do.
Are they giving non-famous people a false moment of "OMIGOSH SOMEONE NOTICED MY ART" when they see "1 Feedback" message, only to realize that it's just ANOTHER "Thanks for fav" comment? YES, FOR CRYING OUT LOUD, YES!
So please, be understanding, and use comments to talk about my art, me, or to leave any questions that may bother you, but PLEASE, DO NOT THANK ME FOR FAVES. Okay? Please?
(Granted, it's been a long while since I last had this problem, but the point still stands. I am interested in reading comments regarding my deviations, art or something like that, not this unoriginal, copypasted "Thx 4 fav" stuff.)