This is addressed (in length) in the tie-in novel 'Man of Steel - The Early Years' which focuses heavily on Clark's upbringing. In brief, he's been raised by the Kents to focus on living his life as a good person. His father advises him to keep his true identity a secret for Clark's own sake. Clark obeys, but this leaves him feeling depressed and directionless. Once he starts doing superhero stuff, he instantly feels better.
He remembered how right it had felt to rescue Susie from the fire, and even Whitney from that flood years ago. Human or otherwise, he had a chance to make a difference on Earth, no matter what world he had been born on.His head was full of questions, but one thing was certain. Clark wanted to be the hero his parents had raised him to be.
Note also that being heroic Is something that he views as his moral imperative.
In time, he had learned the truth about himself: that he was the Last Son of Krypton, sent to Earth by his birth parents, Jor-El and Lara, so that he would have a chance for life on a new world. The "S" on his chest, which now stood for Superman, was also the crest of his Kryptonian family. He honored their sacrifice by doing his best every day to protect the people of his adopted planet. Krypton was gone, but he could still fight for truth and justice on Earth
