Remember this, when trying to find a valid lewis structure, every single atom must have a total of 8 shared electrons (full octet). Since I only have 4 hydrogens to work with, two on each carbon, I assumed a double bond in the middle. This way, every atom has a full octet, except for hydrogen which is most stable with just two electrons. If there were just a single bond in between the carbons, each carbon would only have a total of 6 shared electrons which violates the octet rule. So a double bond is a must. This is ethylene by the way.
