why pz-pz bond is sigma bond but px-px, py-py px-py bonds are pi bonds ?
All the three orbitals (px-px, py-py and pz-pz) can form sigma bonds. But generally, z axis is taken as the internuclear axis, so two pz orbitals combine by head on overlap and form sigma bond while two px and two py orbitals overlap sideways and thus form pi bond. There is no interaction between px and py orbitals.