With its seeming equation of virtue and beauty, The Picture of Dorian Gray looks like a fable. For the purposes of the story, everyone accepts that Dorian Gray must be good because he is beautiful, at least for a while. But I think that Wilde is really trying to puncture that attitude by a sort of reductio ad absurdum.
Lord Henry Wotton, who leads Dorian down the path to ruin, is an engaging character, and it seems that the narrator picks up Henry's manner. Most of the epigrams that we hear from this novel come from Henry's mouth. ("People nowadays know the price of everything and the value of nothing," "The only way to get rid of a temptation is to yield to it," etc). It's easy to be seduced by Henry's manner -- Basil Hallward, his foil, is a comparatively a bore.
But ultimately, Henry's path is barren. He believes in beauty only, with no connection to the value of the underlying object. One senses that he might even approve of Dorian's cruel acts, since Dorian has the picture to absorb their consequences. Wilde makes Henry so entertaining, because if we were instantly repelled by him, we would never think that his attitude could touch us, that we could be cruel like Dorian.