Here's why:

* Stability: Atoms are most stable when their outermost electron shell is filled. A full outer shell means the atom is less likely to react with other atoms.

* Octet Rule: Most atoms strive to achieve a stable configuration of eight electrons in their outermost shell (except for hydrogen and helium, which only need two electrons). This is known as the octet rule.

* Bonding: Atoms achieve a full outer shell by forming chemical bonds with other atoms. This can involve sharing electrons (covalent bonding), transferring electrons (ionic bonding), or a combination of both.

Let me know if you'd like more details on any of these concepts!