Why is giving someone the middle finger a derogatory gesture?

I honestly have always thought this was a bit silly, however in high tension times, mine is up in the air. However when you think about, your offering something to a person you are not liking very much at the moment. Instead of them being happy, they tend to get more mad. It's strange isn't it. I wonder who started it and deemed it to be a way to tick someone off? Strange.