What does the reactivity of an atom arise from?
From a lack of stability
The second law of thermodynamics says that everything in the universe moves from order to disorder.
A corollary of the second law is that everything moves to state of greater stability. Atoms will always move from an unstable structure to a more stable structure.
A atom like Sodium is unstable electronically. Sodium has one valance electron in the outer shell. This is an unstable structure the Sodium atom will react to remove the unstable electron and achieve greater stability. Atoms react chemically to achieve more stable less ordered systems.
An atom like Carbon 14 is unstable in its nucleus. Carbon 14 will react to achieve a more stable nuclear structure. The emission of a beta particle will cause the Carbon 14 to become the more stable Nitrogen 14.
Either chemically or nuclear atoms will move to achieve a more stable structure.