Who is asking: Student
Decartes was not responsible for choosing counterclockwise as the positive direction. In fact, he did not use a y-axis -- he just used a horizontal x-axis as his coordinate axis and had up as the natural positive direction for the values of his functions.
In the Quandries and Queries data base there is a note called "why counterclockwise?" that addresses the counterclockwise question.
Although one can't be certain why counterclockwise was chosen as the positive direction, the guess is that it is related to the way the solar system moves when viewed from a point in space above the Earth's North Pole -- the planets all move in the counterclockwise direction, and that's the direction in which the Earth spins about its axis.Cheers,