It's so obvious! by the trailer alone, it shows how people are demonizing superman for being a "god". The way in which people are shouting that Superman is a false god, he's not natural. Over the course of human history Superman will be cast out because of the destruction he caused in Man of Steel. It's because of him millions of people died, and the people of earth and Metropolis will place the blame on Superman.
So the question remains, how can Superman redeem himself?
Superman is going to stage the fight with batman, to let batman win to show the world he is not a god. By doing this, it will show the world that he is not a god, and should not be treated as such, and will give Batman more credibility as a human.
That is why Superman will lose in Batman VS Superman: Dawn of Justice.
PS: The title also alludes to the fact that this movie will be before the Justice League, and will possibly lead into it.