Here's why:

* Inertia is the tendency of an object to resist changes in its state of motion. This means an object at rest wants to stay at rest, and an object in motion wants to stay in motion at a constant speed and direction.

* Mass is the measure of an object's inertia. The more massive an object is, the more inertia it has, and the harder it is to change its motion.

So, when we talk about motion being resisted, we're referring to the inherent resistance to changes in motion that all objects possess due to their inertia.