Why do sigma bonds form?
Sigma bonds form because they allow both atoms to complete their valence shells and go to a lower energy level.
When two orbitals overlap linearly to form a σ bond, the shared electrons spend most of their time between the two nuclei. This is the most stable arrangement. It leads to the lowest possible energy level.
Atoms tend to form as many sigma bonds as possible to get to a lower energy level. Thus, when C and H atoms come together to form ethane (C₂H₆), they form σ bonds as below.
The C-H and C-C σ bonds all have the shared electrons in the region between the two nuclei.
Sigma bonds are more stable than π bonds. In ethene, for example, the π electrons are "offside" from the bond axis. Their attraction to the two nuclei is not as strong as if they were directly between the nuclei.