Hi /u/Drippidy!

In order to explain this fact, we have to understand binding energy:

Probably the most famous equation in physics is E=mc². It tells us that mass is a form of energy and can, therefore, be transformed into other forms of energy (just like p.ex. movement energy can be transformed into thermal energy).

Atomic nuclei are made up of protons and neutrons. Protons hold positive charge and therefore repel each other. The reason why atomic nuclei can nevertheless be stable is, that they are held together by short-range attractive forces called the strong force and the weak force. If this sound confusing, your take-away should be that there are two kinds of forces in atomic nuclei, one kind is attractive and the other is replant. This pull-and-push game means, that there is one combination of protons and neutrons that form the most tightly bound nucleus: Iron. In iron, the attractive forces win against the repellent force by the largest margin, so to speak, forming the most tightly bound nucleus. All other combinations, i.e. nuclei that have both fewer or more protons and neutrons in the core, are less tightly bound than iron.

Therefore, very light nuclei, which have fewer particles in the nucleus than iron, get more stable by gaining protons and neutrons, while nuclei that are larger than iron get more stable by losing protons or neutrons. In physics, being stable is always associated with minimizing your potential energy, so the closer nuclei are to iron, the lower is their energy level. As iron is the most tightly bound nucleus, it is the most stable configuration. Therefore, fusing iron into heavier elements requires considerable energy to be put into the system, rather than gaining energy through fusion as is the case for lighter elements.