The author is expressing the ideology of hedonism, where the purpose of life is to experience everything without any limits or boundaries.
"Dorian Gray does not balance his love of beauty with a sense of morality, he sinks into selfish behavior."
"Wilde was an admirer of Pater, and it was Wilde who later became the representative figure of Aestheticism. Pater’s influence on The Picture of Dorian Gray was profound."
Since Oscar Wilde was a follower of Walter Pater, he wrote a book celebrating the beliefs of his mentor.
"Pater emphasized the fleeting nature of life and argued that the most important thing was to relish the exquisite sensations life brings, especially those stimulated by a work of art. The aim was to be fully present and to live vividly in each passing moment."
Even though Dorian does not age or bear any evidence of a conscience, his portrait does, therefore, I believe that Wilde was making a comment on the risk of the hedonistic lifestyle, recognizing the consequences. Although on the outside, the individual would appear to enjoy the indulgence in all pleasures, inside, there would be scars and all the signs of sin present in the ugliness that is well hidden.
Oscar Wilde, as an artist will do, created a work of art by writing this novel. As he states in the Preface, "Thought and language are to the artist instruments of an art." He also states that "There is no such thing as a moral or an immoral book. Books are well written, or badly written. That is all." He suggests, then, that his book is amoral; this is a curious reflection on the amorality of the characters found within, and, in the case of Dorian, his transition to amorality, and the repercussions of that transition. Even Dorian's last name suggests that nothing is black or white, but that life itself reflects subtle color shadings found in his picture.
I believe there are many reasons. I believe he meant it as a metaphor, that a picture can be painted, or a book written, one so beautiful or artfully done that it is immortal in the viewers eyes, but the picture or book itself could erode over time. Events unforseen could destroy its original beauty, and if the artist saw what his masterpiece has become, then he would theoretically die.