Because he continues to take a logical, rational approach to important issues. In quoting from the President's recent speeches on the situation in Iraq, Sullivan commented:
"Senator McCain has rightly pointed out that we have one president for the next three years; and we are at war. Criticizing him is fine; but rooting for him to fail isn't. I fear it may be too late to rescue Iraq from disintegration. I hope it isn't. No one knows right now. But explaining to Americans the details of what is going on, and not hiding from them the truth of the dangers and trials ahead, is essential to victory."
A link to Andrew Sullivan's blog is at right; you can read the whole piece there.