And "Smallville" very literally and obviously means a small town (for an allegory, it's big and unsubtle enough to trip over). So he grows up in a small town in the heart of the US, brought up by simple and honest country folk. The implication is obvious. Here's an immigrant, naturalised as an American by imbibing the best folk values of middle America. And even when he goes to work in the big city (named Metropolis - the allegory-spinners are really hitting us over the head with a sledgehammer here), he still doesn't lose his small-town values. He remains modest and well-mannered and more than a little socially awkward. But he then starts to do enormous good, and the stories are all about his wonderful deeds and exploits. In other words, the immigrant makes good -- big time.
Why do I like Superman so much? Because I identify with him. No, I have no super-powers (not any that I'm conscious of, anyway), but I see in him an introvert like myself. People only know me by my accomplishments. They think about me in terms of where I have studied or what kind of work I do, but none of that is me. No one knows my inner self. I keep that strictly to myself. Similarly, it is a mistake to be misled by our hero's flashy costume and all those thrilling action sequences. They do not define him. In his heart, Superman is not a flamboyant super-hero. He is private and self-effacing. In fact, the costumed superhero is Superman's secret identity behind which he hides. His true persona is Clark Kent. I identify with Clark Kent.
There is one very powerful (dare I say "spiritual") aspect to Superman's character. He does not kill. He is merciful even to enemies who he knows would gladly kill him given half a chance. He's a far better man than I am in that respect. In his place, I don't think I'd spare someone who I knew was going to try to kill me at the next available opportunity. [Thirty years after I was introduced to Superman, I came across another such impossibly noble fictional character, and that was The Doctor.]
I found this very nice summarisation of Superman's character from one of the comics I recently read, because it depicts both his own nature as well as the expectations that people begin to have from him.