Why do disproportionation reactions take place? I want to know what are the conditions when disproportionation reaction is feasible. Is it just because of stability factors or it has other dimensions to it. As an example:
$$\ce{3HNO2 -&gt;  HNO3 + 2NO + H2O}$$

Nitrous acid disproportionates to nitric acid and nitrogen monoxide. Here stability of +5 in nitric acid is understandable but how +2 in the monoxide is stable if the disproportionation takes place to gain stability.

Am I ignoring some other factors like pH or hydration enthalpy. I would be very thankful if someone gives me some general ideas on these reactions.
"Stability" is not really a driving force of a reaction. That would be the change in Gibbs Free Energy or ∆G. That takes into account both entropy and enthalpy, as well as concentrations of reactants and products, and temperature.

If you haven't yet discussed these in class, you should be able to read about these concepts by looking up "Gibbs Free Energy" in your textbook's index.