Do you read them?
I do if I ever browse through DA (which is not much). I don't care for the ridiculous amounts of sonic and ponies fanart (good on ya for liking something more than anyone should I guess. You're one of thousands. Why don't you come up with your own thing? Wait, that requires thinking. Nevermind). I'll occasionally look through the original digital art and photography.
First of all, if you favorite a picture, do you leave a comment? Is that comment more than, "I like this"? If it's not, don't bother leaving a comment. That's what "favoriting" the picture implies. Though, who regularly looks through their favorites? (I only have 5 rotating on my page as of right now so that makes it pretty easy) For the people who just browse, favorite everything they see that they like, and never go through their favorites again, what's the point?
Do you think about your comments? Are you going to be the third, tenth, hundredth person to say "That looks cool"? Do you honestly think that comment is worth the time spent by artist who drew the picture? Do you think that you could phrase a critique that isn't offensive? Maybe you should try. As someone who draws, I'd appreciate a "That looks interesting. Have you considered ____?" or "Did you notice ___? It looks weird to me." I believe everyone has the ability to see if something is "off" whether they're correct or not is a different matter but the important thing is artists should always want to be better and if you make them think about something more, in the long run it should yield better results... so long as you're not wrong or they ignore you completely if you are indeed correct.
As I've been drawing for a while now, I'm less and less interested in fanart. Not that it can't be well done or I don't like it or anything. Some of the Touhou fanart is fantastic and I don't use any of my pictures as desktop wallpapers but "favoriting" fanart is like favoriting a coloring book page. You wouldn't do that, right? There was no artistic input. There's maybe style input but that just how someone colored the coloring book page. Fanart can be a tool to practice proportions, for example, but whatever original thing the artist draws is vastly more important to him/her than the fanart can ever be. (It is for me at least and I feel this is objectively correct for all artists).
So, browse some original art, take more than 10 seconds to click "favorite" and leave more than some drop in the bucket, "durrr... it luks gud" comment. Thanks.