It's not losing itself that's aggravating; it's the whole experience of slipping once, then having every thing you try to do to catch up or even the lane out for the next 10-15 minutes completely fail because the game is designed to give the person who killed you numerical superiority. Then their ALLIES prey on you and make it worse, and you end up wasting the next 40 minutes of your day in a game where no effort you give is worth a damn because you don't have the hard numbers to do anything worthwhile except maybe MAYBE soak up a death in place of one of your allies.
Being on the losing side in this game makes you a worthless pile of trash for 45 minutes of your life and it aggravates me.