Here's why:

* Kepler's Second Law: This law states that a planet sweeps out equal areas in equal times. This means that a planet travels faster when it's closer to the sun and slower when it's farther away.

* Gravity and Conservation of Energy: The sun's gravitational pull is strongest near the sun. To maintain a stable orbit, a planet closer to the sun must move faster to counteract the stronger pull. Imagine a ball tied to a string being swung around: the faster the ball spins, the tighter the string can be pulled.

* Conservation of Angular Momentum: This law states that the angular momentum of a system remains constant. A planet's angular momentum depends on its mass, velocity, and distance from the sun. As a planet moves closer to the sun, its distance decreases, so its velocity must increase to keep its angular momentum constant.

Think of it like this: a planet's orbital speed is a balance between the sun's gravitational pull and the planet's own inertia. The closer the planet, the stronger the pull, requiring a faster speed to maintain its orbit.