MoS shows us a Superman who fought for the little guy and defended even the bullies before he was even Superman. He saved children from drowning, oil rig workers from fiery death, and a young woman from sexual harassment. It showed us a Superman who risked exposure by saving the life of a well-known journalist. Superman saved the lives of the citizens of Smallville and the soldiers who do not yet know that he was "not [their] enemy." When he learned that the World Engine would weaken him, Superman's response was to say, "I'm not about to let that stop me from trying." He risked his life to stop the terraforming that was ravaging Metropolis on the other side of the globe. His efforts saved everyone, including Perry, Jenny, and Steve.
Moving beyond MoS to BvS, Clark fought for the poor and disenfranchised in Gotham, including even the city's criminals who were living in fear of The Batman's reign of terror. Superman fought for a little girl dying in a fire in Juarez. He even saved the life of Lex Luthor who would have died at the fist of his own abomination had Superman not intervened.
Superman is fighting for his world -- our world -- to be a symbol of hope and a force for good. It's a cause he was willing to die for, and he did. Wonder Woman AND Man of Steel were both beautiful and powerful origin stories for their titular heroes because they both revealed the heart of their heroes through their actions and their choices.