I was just discussing this with a friend the other day. One, I think that your answer carol is one part. "Untitled" gives the viewer no preconcieved framework from which to view the piece. Also, sometimes the artist is too close to the work emotionally and doesn't want to reveal what they were feeling, or perhaps aren't sure what they are feeling so aren't sure how to express it in words. What I hate is when it seems lazy. Like the artist just didn't feel like coming up with something. That signifies that they don't care enough.
Benjamin
