why is that ketones do not react with tollens reagent but aldehydes do?

Tollen’s reagent is one of the reagents which is used to distinguish aldehydes from ketones.The reaction of aldehydes with Tollen’s reagent is an oxidation reaction.Aldehydes are oxidised easily but ketones do not.
Tollen’s reagent is an ammonical silver nitrate solution.When an aldehyde is treated with this reagent,silver ions are reduced to metallic siver and a bright silver mirror is produced on the inner sides of the test-tube.This oxidation requires an alkaline medium.
RCHO + 2[Ag(NH₃)₂]+ + 3OH- ---------> RCOO- + 2Ag + 2H₂O + 4NH₃
The reason for this difference is that an aldehyde molecule has H-atom (RCHO),due to which they can easily oxidise but in case of ketones(RCOR’) they lack H-atom so they resist to oxidise and only very strong oxidising agent like potassium permagnate can only oxidise them.