I've always seen it as I'm saying "no" to alleviate any doubt the other person had that I did not agree with their statement.
The conversation might go: A. I really think "OK Computer" is the best Radiohead album. B. No, totally!
But the implication is more like: A. I think "OK Computer" is the best Radiohead album. Am I crazy for not picking "Kid A" or "In Rainbows?" B. No, I don't think you're crazy for liking "OK Computer" the best. I totally agree with you.
Does anybody else see it this way? (Please respond with "No, totally!" (It's okay if you don't like "OK Computer"))