Because I find him to be a very repetitive writer. Or more accurately: he seems to continually promise that he is about to say something new. After all, his 'materials' (the constant references to books, films, songs, poems etc) are very diverse. I draw the inference, therefore, that he has something to say about lots of topics! But then, no, what is animating all of his references, all of his bits of collage, is the same essential point: 'what exists is pure variation, evaluation must be creation...' Perhaps the headache is more a sense of fatigue.

Also, I have various study-related projects that mean that I HAVE to read Deleuze. And everyone knows that that's the best way to take the fun out of something.