The term inertia is used in a variety of different contexts: physics use it to describe features of a particular object, but you can also call a person "inert" if you want to call them out because of their lethargic or even lazy behavior. Whether we are talking about an inanimate object or a living person, they both have one thing in common: they refuse to change the state in which they are not moving.
What Is The Natural State Of An Object?
Inertia can most easily be explained as a feature that every object has, and it is described as resistance to movement. If you want to move anything, even yourself, you need to apply some sort of energy into that process. The reason why energy is a critical concept we need to link with inertia is that, as it was stated by the father of physics, Sir Isaac Newton, all objects in the universe want to preserve their state in any particular moment. This concept is known as the first law of physics.
Now, the question you may ask - why do all objects "suffer" from inertia? The answer is linked to the second law that Newton postulated. The objects are inert because they have mass. Let us look at this formula: F=ma. This means that any kind of force (F) is a result of the interaction between mass (m) and acceleration. How does this link with inertia exactly?
Mass And Inertia
Imagine that you are applying the same amount of force (which is in our case energy), to two different objects you wish to move, and those two objects have a different mass. The bigger the mass of an object is, the harder it becomes for that object to move, which perfectly describes the state of inertia.
If you have a basketball lying on flat ground, you can easily push it with one hand. If you have a car standing on that same ground, and if the handbrake is not engaged, there is no chance you will be able to push it with just one hand. This means that a car is more inert than a ball and that the amount of force you need to apply to change its acceleration is far greater than the one you need to use with a ball. So, the most simple answer to the question in the title would be: mass causes inertia, and it is a feature of all objects.
Gravity And Inertia
But, there is something that affects how we perceive mass, and it is an undeniable feature of the known universe, an invisible force that seems to hold everything together. Of course, we are talking about gravity. Gravity is a force that, in essence, allows objects to have mass in the first place. If there were no gravity, objects would be weightless, and it is hard even to imagine a world in which everything has a desire to float, and there is no force that is pulling it back to the ground (or to another planet).
However, this does not mean that the objects we observe floating in space can avoid inertia. Newton said that inertia is "vis insita" of all matter, which means that the resistance to move can be seen anywhere. Even in zero gravity situations, you need to apply some force if you want to move an object. This force will be significantly smaller (again, depending on how far the object is from the source of gravity) than on the ground, but still, absolutely nothing wishes to move on its own.
In conclusion, inertia exists because of gravity and the effect this force has on all objects we find in space. The stronger the gravity is, the bigger the mass of an object becomes, which results in an increase of force that is needed to move an object from its inherently inert state.