Here's why:
* Newton's Third Law: For every action, there is an equal and opposite reaction. This law applies to all forces, including gravity.
* Gravity is a Mutual Force: Gravity isn't just something the Earth experiences from the Sun. It's a mutual attraction between any two objects with mass. The Earth pulls on the Sun just as strongly as the Sun pulls on the Earth.
* Equal and Opposite: The force of the Earth pulling on the Sun is exactly equal in magnitude to the force of the Sun pulling on the Earth, but in the opposite direction.
Important Note: While the forces are equal, their effects are not. The Sun is much more massive than the Earth, so the Earth's pull on the Sun doesn't cause the Sun to move noticeably. However, the Sun's pull on the Earth is what keeps the Earth in its orbit.
