As artists, most of us have been around people who have no artistic inclinations, or are just beginning, or haven't had as much practice, and they basically fall all over themselves to tell you how great you are. It's pretty gratifying the first hundred times and then it gets a bit.. monotonous. I know that sounds rude and self-absorbed, but when you hear all these compliments without any kind of action to back it up, you kind of filter it out after a while.
Case in point:
I've done a few art shows. They're not my favorite thing in the world because, while I have no objection to showing my work to the masses, I have a hard time communicating with strangers. I am NOT a salesperson and I won't pretend to be one either. I have a low bullshit tolerance and can't allow myself to bullshit other people as a result.
At these shows, I've had people come up to my booth, or wall space, or table, and tell me how talented I am, how much they love my work, and then WALK AWAY. What that says to me? You're full of shit. If you loved it so much, why not help me out and, oh, I don't know? BUY IT. At the very least you could give my information to someone else who might, right? This isn't to say that everyone should buy work from every artist they like. I know cash isn't easy to come by (I know this VERY WELL), but when this keeps happening over and over again, it makes the artist (you, me) believe that these people are completely disingenuine.
What does this have to do with favorites?
Look at it this way: when you give someone a comment, it's a recorded compliment. It's nice. It certainly took more time than hitting the favorite button, but other than that, most people other than the artist will never look at it. On the other hand, when you favorite someone's work, you're saying that not only do you like it, but you like it enough to promote it on your own page. That promotion is akin to digital buying. You're doing me a favor by giving me more exposure. In turn, I'll give you a thanks or a llama, and check out your gallery to see if I can return the favor.
This is why I don't respond to most comments (unless they're questions or demonstrate exceptional ass-kissery). I don't do it to offend you, and for the record, I DO read them. I just don't take them as seriously as I will a favorite.