Dorian Gray is so many opposites at once that we can't really try to understand them all. He's good, he's bad; he's beautiful, he's hideous; he's perfect, he's terribly flawed. Most importantly, he's literally two things at once – he's himself, a living, breathing human being, and he's also the portrait, a visible reflection of the state of his soul.
You know, eternal youth and beauty sounds like pretty good deal. However, it definitely falls under the category of "totally, absolutely, way too good to be true." The thing is, most of us realize that there's more to life than a pretty face. But Dorian Gray's trouble is that he only values himself as an ornamental creature. He doesn't recognize his own beauty until he sees it reflected in Basil's portrait, and, once he does, it's all downhill.
Dorian immediately identifies his beauty as his only worthwhile quality. He doesn't realize that his true value lies in his innocence and purity, which lend his physical beauty a kind of magical appeal. As long as he remains beautiful on the outside, he doesn't think it matters what happens under the surface. And, for a while, it seems like he's right. After all, he does lead kind of a charmed life for almost twenty years.
The thing is, Dorian's not the only one with a warped view of true beauty. He's playing upon the fact that most people, like him, tend to equate outer beauty with inner beauty, and as long as he can continue to fool them into thinking he's a saint, he can sin all he wants. Sure, sinister rumors abound about him, but people who actually meet him can't believe the stories. His picture-perfect good looks are the only thing that keep him going; he lives purely for physical pleasure, and delights in his own aesthetic qualities.
The Dorian Gray we see in Basil's brand new painting is the original Dorian Gray – he starts as a young, pure, beautiful thing, whose utter innocence is literally painted on his face. However, as the story goes on, the painting shows what lies beneath Dorian's surface: a monster. All of his crimes and misdemeanors are reflected in the aging, malevolent face of his portrait, and here, we see what Dorian has really become. However, as long as he retains his physical beauty, this hidden true self doesn't seem to matter.
Dorian takes a kind of perverse delight in seeing just how depraved and corrupt his secret soul is. When he's anxious about the portrait, it's not because of guilt; rather, it's due to fear that people will discover his secret. And the most terrible secret of all? It's not the painting itself, but what Dorian's treatment of the painting represents. He's totally aware of how evil he is, but he doesn't want to do anything about it. He likes being evil – he loves being evil. At the end of the novel, when Dorian briefly thinks that he should try and turn back the moral dial to "Good," we know as well as Wilde does that it's impossible. All this time, Dorian has had various points at which he could change, and he never does. By the end of the novel, it's too late – for all his innocent, golden beauty, the real Dorian is bad to the bone.