He always has that kind of grin on pictures. And it's no wonder if you know all the hardships and humiliations he's been through. You can easily find on the net a New York Times article about him, or one about the book, called Hard Drivin', which the film is based upon.
Of all the photos of Scott, the one I like best is this one, where you don't actually see him. Just his mother, wife and kids watching him race.