Is there an off Switch for emotions?

The above question is more than legitimate in my eyes. Today was overall a great day at work. I was able to get all my work done in a timely manner with only minor issues to work through that provided some hiccups. However, it was not to last. Despite the decent performance I gave at my job, I forgot that I work with a complete asshole. For some reason, this guy thinks that he has to bitch and moan about my performance at work, as well as my hygiene. I will be the first to admit that I'm not perfect, and that there is room for improvement in both of those areas, but this guy is way over the line.

I asked him straight out today what his problem with me was, and this is the answer I received:

1. My work is half-assed

2. I know it

3. I don't care that my work is half-assed

It went on for longer than that, but those are the three main things I remember that he told me. Now, let me be clear. I don't care whether this joker thinks my work is bad, because I know it's not, and I was just shown an evaluation that showed my performance, which was already satisfactory, improved. I know he's wrong. I know my work is good, and that his opinion doesn't matter. So why is it that I can't seem to avoid letting this stuff get to me? Maybe it's because I just don't understand what the motivation is for someone to act that way towards people. Why the hate? Why the dislike? What does it matter whether someone else is performing poorly if you yourself are performing well?

Moreover, specific to this issue, it seems that I'm the one he immediately blames when something either doesn't get done, or goes wrong.

the issue, though, isn't whether he thinks I don't care about my work. The issue is the fact that even though I know he's wrong, I can't help but let him get to me a little. I lasted most of the day without snapping, but towards the end I did shout at him and storm out. However, since I was ready to throw this jerk up against the wall and snap his neck in two, I think that was the better outcome.

It's times like these that I wish I could be like Commander Data of Star Trek and just flip a switch to turn my emotions off, and then back on again. But, as I'm not lucky enough to be an android, I have to deal with it. Still, it would be nice if I could just do mjy work and have people leave me the hell alone.