Pi Bonds in Double Bonds

[305920501]

Why do all double bonds have a pi bond and a sigma bond? I understand the overlapping that represents a sigma bond, but why wouldn't there be two sigma bonds-- how do they interact through a pi bond?

[205873468]

All double bonds contain both sigma and pi bonds because of the hybridization qualities. the sp2 orbital causes overlap (sigma bond) and the orbitals that are unhybridized overlap as well. The overlap in the p orbital is why there is a pi bond.