Not really knowing any of the story...here's my thoughts:
A big clue to your boss's motivation is his use of "negativity." Personally, I find that no one ever expresses negativity in a positive manner, unless of course there's a sarcastic undertone to the topic. The guy is terribly insecure. By focusing on what's wrong with you, he's hoping that you'll be too preoccupied to decide that he's merely a larva in a business-suit. Chances are, this runs much deeper than the job at hand. He most certainly has bigger issues outside the job and because of this he's probably suffering more than you.
When something negative comes along, rather than reacting, engaging, or indulging in that negativity, consciously move your mind to something positive. For example, if you have a negative thought, replace it with a very positive one. In much the same way that your body needs to be worked out in order to become strong, your mind is like a muscle - you have to work it out in order to strengthen it! And eliminating negative thoughts and emotions from your mind is like physical exercise - it's a good habit to get into! Eventually, you will find that you feel better and happier throughout the day, because you will not be allowing yourself to be brought down by any negativity you may encounter. Then at the end of the day, you will discover that you have more energy for the evening. Zen Beast has spoken.LOL