Carbon and oxygen are both nonmetals. That means niether of them is willing to lose electrons. That means that they both will try to either gain or share. Since the only way to gain is for someone else to lose, the only solution is for them both t o share. C has 4 valence electrons and needs 4 more. O has 6 valance electrons and needs 2 more. That means that it takes two oxygens to share with the carbon to supply the needed 4 electrons. Each oxygen shares 2 electrons with the carbon. THe result is formation of TWO bonds between each oxygen and the carbon.
