The nucleus of tritium (hydrogen-3, $\ce{^3H})$ contains one proton and two neutrons. Nothing repulsive is acting on the proton that can make the nucleus unstable. Then why does it decay?
I suspect the answer is related to the internal structure of the protons and neutrons. This applies as well to the one with the greater number of neutrons than it should be. My question applies to those isotopes as well.