Useful feedback. Trying to get better at explaining things like this, so thanks.

Some other good answers already, but this is ELI5, so let me anthropomorphise everything for you. Lets swap the concept of energy with the concept of money.

Hydrogen is a very rich atom. So, it has the abillity to do as it pleases. It can afford to fuse with another hydrogen (actually more than one hydrogen atom involved here...) to make helium. The money the hydrogen atoms pay is released into the surroundings, and so the helium atom now cannot afford to become hydrogen again. Helium can still afford to fuse into carbon and oxygen, which can afford to fuse into neon, then silicon, and then iron.

Iron is now the poorest element. It cannot afford the cost to return to silicom, and it cannot afford to fuse to heavier elements. On its own, it's stuck. If iron wanted to change, it is completely reliant on the its environment to provide money for it. Not even the core of a star can afford the cost, but a collapsing star absolutely can, which is how we believe heavier elements are made.

Finally, you may ask "just because helium can afford to fuse, why does it?". The answer is because helium never wants anything, it has no will. If it's possible, then helium has a non-zero probabillity of doing it. Helium cannot spontaneously become hydrogen, but it can fuse to heavier elements. Given enough time, it'll happen.

Hope that helps!