Hybridization is the intermixing of atomic orbitals to form hybrid orbitals of same energy. They are degenerate and the filling of electrons occur according to Hund's rule and Aufbau principle. These hybrid orbitals overlap with orbitals of the bonding atom to make sigma bonds. Sigma bonds have only one overlapping region and that can be done easily with the hybrid orbitals. Unlike atomic orbitals, hybrid orbitals have a thick electron density and a thin electron density. Thick electron density helps to increase the extent of overlapping during sigma bond formation. However, because of the thin electron density the hybrid orbitals can not do side wise overlapping and create two overlapping region required for pi-bond. Therefore hybrid orbitals can not form pi-bond.