Here's why:

* Kepler's Laws of Planetary Motion: Kepler's second law states that a planet sweeps out equal areas in equal times. This means that when a planet is closer to the sun, it has to move faster to cover the same area in the same time as when it's farther away.

* Conservation of Angular Momentum: A planet's angular momentum (a measure of its rotational inertia) remains constant. As a planet gets closer to the sun, its radius of orbit decreases. To keep angular momentum constant, its speed must increase.

* Gravitational Force: The sun's gravitational pull is stronger closer to it. This stronger pull requires a higher orbital speed to keep the planet from falling into the sun.

Think of it like a spinning ice skater: When they pull their arms in, they spin faster. The same principle applies to planets: as they get closer to the sun, their orbital speed increases.