I've read recently that the mass of a proton is mostly not given by the Higgs mechanism. But rather it's given by the energy of quarks moving around inside the proton and gluons and other internal sources of energy.
Obviously mass and energy are equivalent, so I'm assuming we can calculate the rest mass of a proton from the energy it holds. But wondering this made me question why exactly you get "inertia", or resistance to acceleration, at all from energy.
Why are things harder to move when they have more energy? Is there some process by which things with a lot of energy resist changes in motion? I feel like everyone just says $E = mc^2$ without actually asking why we experience the phenomenon of mass and inertia at all?