I found it on Wikipedia. At first, he fixed zero at the stable temperature of a “mixture of ice, water, and salis Armoniaci [transl. ammonium chloride]” and 96 at the human body temperature, but later he would change the lower reference point to water’s freezing point at 32 and still later the upper one to the boiling point of water at 212. So it has always been pretty arbitrary.
Edit: But I will agree that the scale of zero to one hundred does correspond more closely to how warm humans feel.