He briefly talks about Kasparov here as they were training together at the time. They later broke up.

I wonder if some of the conflict they felt was was really Magnus's more organic, natural approach clashing with Kasparov's more like systematic and rigid type of training, which he no doubt inherited from Botvinnik. This isn't to say Magnus doesn't work hard at his game but on a chess approach scale of, say, Capablanca to Botvinnik, he would very much tilt towards the former.

edit: Actually he specifically talks about that; somehow I skipped it upon first reading.