This is due to Kepler's Second Law of Planetary Motion, which 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 amount of time as when it's farther away.

Think of it like this: imagine a planet's orbit as a stretched-out oval. When the planet is close to the Sun, it's like it's on the "skinny" part of the oval. To cover the same area as when it's on the "wide" part, it has to move faster.