If you’ve taken a little chemistry you probably know that the electrons in an atom “stack up” in energy levels. The same is true for the protons and neutrons inside the nucleus of the atom. In a stable atom the energy in the proton stack will be about the same as the energy in the neutron stack. If they’re unequal, then a neutron will turn into a proton ( decay), or a proton will turn into a neutron ( decay) to even out the levels. Forcing protons together takes a lot of energy (likes repel), so after hydrogen, proton energy levels get higher, quicker, than than neutron energy levels.