Here's why:

* Inertia is the tendency of an object to resist changes in its motion. This means it wants to stay at rest if it's at rest, and it wants to keep moving at a constant speed and direction if it's already in motion.

* Mass is a measure of how much matter an object contains. More mass means there are more particles that need to be moved.

Think of it like this:

Imagine pushing a small, lightweight ball and then pushing a large, heavy boulder. Which one is easier to get moving? The small ball, right? That's because it has less inertia. The boulder has more inertia because it has more mass.

In summary:

* More mass = More inertia

* More inertia = More resistance to changes in motion