This paper provides a mechanism-first clarification of the origin of both inertial resistance and gravitational response using only established physical constraints. It shows that when momentum is introduced locally into any finite system, local conservation and finite signal speed require a non-instantaneous internal redistribution. During this causal update, a transient internal momentum gradient necessarily forms, and the observable resistance to change is the physical manifestation of that gradient. The same causal process operates whether the boundary condition is mechanical acceleration or spacetime curvature, explaining the equivalence of inertial and gravitational response without introducing new forces, particles, or mathematics. The result is a necessity-based account that removes inertia and gravity from the category of primitives and grounds them in causal enforcement.