Here's why:

* Electron configuration: Carbon has 4 valence electrons (electrons in its outermost shell). To achieve a stable octet (8 electrons), it needs to gain or share 4 more electrons.

* Covalent bonding: Carbon achieves this stability by sharing its electrons with other atoms through covalent bonds. Each shared pair of electrons counts towards both atoms' octet.

Exceptions:

While carbon usually forms 4 bonds, there are some exceptions:

* Carbocations: These are positively charged carbon ions that only form three bonds due to the loss of one electron.

* Free radicals: Carbon atoms with an unpaired electron also only form three bonds.

* Certain unusual molecules: Some highly reactive or unstable molecules might involve carbon with less than four bonds, but these are not common.

In summary: Carbon usually forms four covalent bonds, but there are some specific circumstances where it can form three.