Sullivan is something of a figure of fun to many of us. He gets a lot of readers but his changes of direction make it possible to answer almost anything he says today by quoting what Sullivan himself said 5 to 10 years ago. He has for instance gone from being a passionate Zionist to being a passionate Israel-hater.
And the passion would seem to be the key to his popularity. I am guessing that most of his many readers are Left-leaning, and Leftists mainline on emotion, regardless of the facts.
There is a big survey of Sullivan's "thinking" (if you can call it that) here that endevours to make some sense out of what drives him -- without success. I think he just likes hearing the sound of his own voice (or its written equivalent). And he would appear to make significant money out of his writings.