The answer to this question is very simple .
There are two major reasons behind it.
1. Catenation :- It is the property by which the carbon can make long chains, branched chains and ring or cylic structural by joining with its own atoms. Due to this it can make a variety of molecules.
2. Tetravalency :- carbon has 4 electrons in its outermost shell, and due to this it is unable to gain or loose electrons.
It only makes the covalent bonds by the sharing of electrons.
It can only share the electrons thus leading to the formation of larger molecules.