Weak bond to strong bond, release energy. Strong bond to weak bond, store energy. Enthalpy is the outside environment, like pressure and temperature, needed per internal state requirements, coarsely being internal voltage and emissivity. As the bonds of internal state strengthen, the environment requires less to maintain that internal state and loosens it's grip on it. Heat then flows from internal state to the environment.
In fusion, hydrogen is converted to helium. One proton and one electron of each hydrogen atom are free to move as they see fit. As temperatures increase, protons and electrons become free of their hydrogen atom internal state bonds. They whip around until two protons together mesh with two electrons. The freedom of movement in the plasma, and the freedom of movement from two separate hydrogen atoms grips into helium as heavier fusion requires a higher temperature. Atoms have an incredibly fine resolution, as fine as something can go. The large amount of them release substantial heat. Add neutrons, the principles of nuclear decay, nature moving heat with mass while leaving internal state properties alone, increases the ability for energy to release.
Instead of thinking of little grains of matter, the internal state is very ephemeral. Before something can be brighter, it needs to be hotter. There's a capacity to hold heat and a latency to transfer heat. This capacity is natures ability to speed up and slow down it's own internal ticking clock to move with least effort. At the smallest scales possible, instead of a little building block, likely its nature's ability to "correct the record". Nothing says nature gets to change the entire environment faster than you can observe it if it's an easier path to move.