[HB]

why is the keto form of acetone is more stable than enol form

[orgopete]

I don't know the molecular orbital argument, but I think you could expect this result for a practical reason. Water is more acidic than methane, electrons are more pulled toward an oxygen nucleus because it has more protons. If the keto and enol form were in equilibrium, the equilibrium would shift toward the less acidic C-H bonds from O-H bonds.

[zsinger]

A ketone formation rxn. is the very strongly entropically favored.  For instance, most nucleophilic substitutions are driven by the RE-formation of the ketone species.  Hope this helps.