Hybridization sp3 and sp2
This can be explained in terms of orbital overlap and therefore bond strength. 

Consider a single carbon in an ethene molecule with two sp3 hybridised carbon atoms. The orbitals on the carbon repel each other and arrange themselves into a tetrahedron. Two of the four sp3 hybrids are used to form sigma bonds with hydrogen atoms. The carbon is then left to share its remaining two sp3 hybridized orbitals with its neighbouring carbon. Draw this system and you can see that two sigma bonds can be formed, though they are "bent" as the sp3 orbitals on the carbon are at a 109.5 degree angle to one another.

Now consider a single sp2 hybridised carbon in an ethene molecules with two sp2 hybridised carbon atoms. Again two orbitals are used to bond with hydrogen. This time we have an sp2 orbital pointing directly at the neighbouring carbon with which to form a strong sigma bond. The remaining p orbital which is orthogonal to the C-C sp bond can also forms a C-C pi bonding interaction.

Draw these orbital systems and compare.


The sp2 hybridised system constructs a strong linear sigma bond.
The sp2 hybridised system better shields the nuclei of the bonding atoms.
The sp2 hybridised system provides more orbital overlap, leading to stronger bonding and lower free energy.