The Solar System showing the Sun, Inner Planets, Asteroid Belt, Outer Planets, the largest object in the Kuiper Belt - Pluto and a comet.
Image courtesy of NASA
Answer: Be careful – the planets rotate counter-clockwise if you view them looking "down" from the northern direction. But if you are looking "up" from the southern direction, they are moving clockwise. The question you may be asking is: why do all the planets rotate in the same direction? It is believed that the entire solar system condensed out of a hot cloud of gases, which formed the sun and the planets and the other matter in the solar system as it cooled. This gas was rotating, which explains two things: first, why are all the planets pretty much in the same plane (because when a cloud of gas rotates, it tends to form a flat disc perpendicular to the axis of rotation); and second, why are all the planets rotating the same way (because the cloud of gas from which they came was all rotating in one direction). The planets rotate in the same direction that the sun is spinning about its axis, which is further evidence that everything came from the original cloud of rotating gas.