Clippy has googly eyes, so when it doesn't understand what I want and blocks me from continuing my task, I think it's an earnest obstructionist idiot. My computer does not have googly eyes so I don't get angry at it because I don't feel it is a person. Also, when it doesn't understand what I want it's not designed to block me from trying things,
I think this is more of a reflection of computer literacy.
When my computer does something wrong, I assume its my fault. When my parent's computers do something wrong, they complain at it. Pretty uselessly.
I guess Clippy just makes people who don't normally over-anthropomorphize computers over-anthropomorphize enough to get annoyed.
From this 2001 article:
I, at least, found this amusing.