Proton Addition

Methyl Orange is an acid-base indicator. In dilute solution at pH greater than 4.4, it is yellow. At pH=3.2 the solution appears red. Draw a structure of the species that is formed at the lower pH of the acid proton adds to the azo nitrogen atom adjacent to the aromatic ring containing the -SO3 group. Why does the proton add to this particular nitrogen when two other nitrogen atoms are available in the molecule?