"Life exists in the universe only because the carbon
atom possesses certain exceptional properties."
– James Jeans, The Mysterious Universe
Carbon is the key elemental building block for all known terrestrial life. It's commonly assumed in astrobiology that it will also provide the basis for most life elsewhere in the universe. The reason for this is not only carbon's ability to form a vast range of large, complicated molecules with itself and other elements, especially hydrogen, oxygen, and nitrogen, but also its unique facility for maintaining the right balance of stability and flexibility in molecular transformations that underlie the dynamic complexity of life. In aqueous systems at temperatures common on Earth, carbon is so superior to any other atom as a polymeric unit, that it has come to be the basis for the structure of biomolecules essential for all basic metabolic processes.
It may be that we find it hard to see viable alternatives to carbon biochemistry because we have no experience of such alternatives. Being carbon-based life-forms ourselves, we may suffer from what's been called carbon chauvinism. On the other hand, scientists have so far discovered nothing in the chemistry of other elements to remotely compare with the millions of organic compounds to which carbon gives rise.