In the mechanism of SeO2 I have seen that a more stable keto form first converts to a less stable enol form and then yields a diketone. I have seen many other reaction where the reactant converts to an unstable form first and then reacts. My doubt is why does the reactant converts itself rather than just simply not react ?
We can think of it an an equilibrium that is disturbed by another, irreversible reaction.
Think of a more familiar reaction. When you try to dissolve magnesium hydroxide in water you get only a little solubility; the dissolved magnesium and hydroxide ions are only a minor species at equilibrium. But when hydrochloric acid is added the acid reacts selectively with the hydroxide ions, neutralizing them; in response more magnesium hydroxide keeps dissolving into the water (to form magnesium and hydroxide ions, the latter then reacting with more acid) until you have a salt solution containing the accumulated magnesium ions and the chloride from the acid. Reaction continues until you've dissolved all of the magnesium hydroxide or neutralized all of the acid.
Similarly, the enol form is only a minor species in the equilibrium, but when you add the selenium dioxide it reacts selectively with the enol form. This draws more of the carbonyl compound into the enol form in the same way that the acid in the example above draws more magnesium hydroxide into its dissolved form. Thereby the reaction keeps going until you run out of one reactant or another.