Clark Kent has many secrets in his life. He has secrets about secrets. One is deadly. No one can know that he really comes from another planet, an alien with powers so far beyond human capability that Pete once nicknamed him 'super-dude'. Only three people do know this: his adopted parents and his best friend. Three other people in his life are desperate to discover the truth of the enigma that is Clark Kent - Lana Lang, Chloe Sullivan and Lex Luthor. Unfortunately for them, Lex has a megalomaniac father and Luthor blood in his veins, Smallville code for spawn of Satan; Chloe is deeply attracted to anything weird (raising the question why she never wondered if Clark was otherworldly) and a dedicated member of the Free Press; and Lana is too much of an attention seeker to let anything in her life pass by without telling someone about it, and so is incapable of keeping secret something so hugely important as that we are not alone in the Universe.
Clark tells no one else his secret as the wrong person could lead the Government Agents to him and that would be the end of everyone's favourite farm boy's peaceful life. He's seen 'ET', he's seen 'Roswell', he's seen every episode of the 'X-Files', he's seen almost every alien orientated movie and show ever made (Gonzo was his favourite on the Muppets). He knows what happens to aliens caught by the Government. Autopsies while they're still alive. Torture. Pain. The deaths of the people he loves. He'd rather eat meteor rocks than go through that.
So he lies by omission every day of his life. There are so many people he loves and lies to at the same time. No one knows his whole story. Kyla, whom he loved, knew most, but not enough. She died because of it. Now he tells no one. It's too dangerous, for the both of them. He'd love to see Chloe's face when he told her. Another daydream on the shelf. That, and seeing Lex roll on the floor laughing until he can't breathe.