I suspect that part of why I feel so drained after GMing is that I am quite the introvert, and GMing is a very extroverted task. For three hours at a time, I'm trying to hold people's attention--and not just hold their attention, but shape and guide their attention as well.
It sounds pretty daunting when I put it that way.
And the other part of it being a task of connecting with people is that I'm not all good at reading the signs that I am connecting with people. So I don't have the instant feedback that I'm doing well or poorly that I have with, say, debugging or fiction-writing. (I'm sure this idea of talking with people providing less feedback than computer-hacking is deeply alien to some people and deeply sensible to some people.) And so I work harder at trying to do things perfectly, and that adds to the strain.
It may not come through in my word choice, but I'm writing this with a much more positive attitude and tone than last night's post. Sure, I'm particularly challenged in this way, but it's a challenge I can work to meet, and it's a challenge that I choose for myself.