Backing the underdog is a concept reserved for two kind of people. Revolutionaries and adrenaline junkies and I am not sure if there is a difference :-)
Backing a team does make it more interesting, it actually grounds the game or rather tethers the game to your emotions and provide you with an opportunity to be an invested in that match. This point of yours reminds me of a post you made a few days ago by the title of 'Facing with no box'.
In that post you asked a very valid question - why do people refuse labels given to them by others and yet then they go ahead and choose labels of their own?
I find the answer of that question in this today's post. Perhaps those labels help us to be invested in something that we believe in. If nothing they else then maybe they simple help ground us to the reality and maybe for one of these reasons we are so vehement in refusing others to give us those labels.