$\beta +$ decay is where a proton gets turned into a neutron and a positron and a neutrino.
However, a neutron is heavier than a proton, so obviously this reaction is endothermic. So then, why does it happen? I've seen an explanation here in the question How can a proton be converted to a neutron via positron emission and yet gain mass?
It describes that the final binding energy of the nucleus increases, thus making it possible by becoming more stable. But what actually causes the reaction to go in the first place? It's like saying this ball will go down the hill because it will lose energy – what gives it the nudge required? Is it something like energy from external gamma rays or something?
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That's true for free neutrons and free protons, which is why free neutrons are unstable against beta decay but free protons aren't. However, the nuclear environment is much more complicated than the vacuum, and when thinking about beta-decay (of either sign) in a heavy nucleus, the free-particle masses aren't the right parameter to consider. What matters is whether the mass of the entire system is increased or decreased by the beta decay.
One hand-waving way$^\dagger$ to think about the energetics in positive beta decay is to remember that protons have positive electrical charge and repel each other. So a nucleus with "too many" protons will have more energy stored in its electric field than a nucleus with the same number of nucleons (protons and neutrons inclusive) but less total positive charge. An observer outside of the nucleus can't distinguish between the energies due to the masses of the constituent particles, the positive (repulsive) energy stored in the electric field, and the negative (attractive) energy of the strong-interaction field which binds the nucleus together --- all of these contributions just add up to make the total mass-energy of the nucleus. If a charged-current weak interaction can decrease this total mass-energy by transforming a constituent neutron into a proton, then that process is exothermic.
$^\dagger$I often describe concepts in nuclear physics using hand-waving analogies and, months or years afterwards, get really interesting clarifications in the comments from other users who are more cautious than I am. I love those and I look forward to them.