Carbon's natural state is to be bound to something, almost anything else.

Methane would have been one of the earliest molecules to form as there was ample hydrogen present to bind with the scant amount of carbon, thirsting for a relationship, to form CH4 in the pre-stellar universe. Upon the first stellar explosions carbon found many more dance partners in the heavier elements and immediately found long-term relationships with most of them.

Carbon defined promiscuity from the outset and with four concurrent bonds to utilize simultaneously also defined "flexibility".