Why does hybridization produce a more stable configuration?

My text, and the other sources I've checked, include information on the effects, and kinds, of hybridized orbitals; however, they do not explain what properties of hybridized bonds conduce greater stability. 


Why does hybridization produce a (lower energy) more stable configuration? 





> 
> What causes orbital hybridization?
> 
> 
> 


Hybridization only occurs in response to bond formation, that is an atom will not just spontaneously hybridize on its own. Further, **an atom will only hybridize if hybridization leads to the formation of a more stable (lower energy) molecule**. 


Nature is very efficient at exploring the energy surface and finding whatever energy minimum is available. For example, ammonia ($\ce{NH3}$) is roughly a tetrahedron with $\ce{H-N-H}$ bond angles around 109.5° (the tetrahedral angle) using (roughly) $\mathrm{sp^3}$ hybridized orbitals. For ammonia, mixing (hybridizing) it's $\mathrm{2s}$ and three $\mathrm{2p}$ orbitals to form four $\mathrm{sp^3}$ hybridized orbitals that are used to form three $\ce{N-H}$ bonds and one lone pair orbital produces a more stable molecule than if it were not hybridized. On the other hand, the analogue phosphine ($\ce{PH3}$) chooses to remain roughly unhybridized, using p-orbitals to form its $\ce{P-H}$ bonds with a resultant $\ce{H-P-H}$ bond angle around 90°. Bond strengths, steric factors, etc. all play into determining whether a hybridized or unhybridized structure will be the more stable.


 4