Dorian Gray is totally young, pure, and beautiful, and his only concerns at this stage seem to be related to staying young, pure, and beautiful. However, this idyll can't last long with Lord Henry Wotton in the works – he seizes upon the young man, and immediately launches into a life-changing lecture about the merits of giving into desire.
It appears for a while that Lord Henry is wrong – Dorian is just loving life, and loving Sibyl Vane. Everything looks peachy, and Dorian seems to have found his own kind of happiness. He's still intrigued by Henry (they hang out just about every day), but his inner innocence resists the corrupting influence of his friend. Dorian's pure love for Sibyl Vane looks like it might be the antidote to the poisonous theories of Lord Henry…however, Sibyl's last performance and her suicide put an abrupt end to this stage.
After Sibyl's death, Dorian is profoundly changed – he goes over to Lord Henry's side and basically becomes a somewhat more evil version of Lord Henry himself. This begins with the yellow book – it "poisons" Dorian's mind, and changes him completely. Dorian gets sketchier and sketchier, as his portrait grows more and more grotesque.
Oh dear. Poor Basil forces Dorian to confront his own evil deeds, and Dorian really doesn't like it. Things just get worse from here on out – Dorian's sucked into a kind of vortex of evil deeds by a combination of factors (Basil's murder, the appearance of James Vane, his own increasing paranoia).
Once Dorian's troubles all subside (James is dead, and nobody knows he killed Basil), he reevaluates his life. His decision to become good again is basically a desire to eliminate the grotesquely ugly Dorian he sees in the portrait. Consequently, his modified wish to keep living as he did and just destroy the portrait is similarly a matter of destroying that other self. However, Dorian doesn't realize that he and the painted Dorian are one and the same – so what begins as an impulse to destroy the evidence of his sins ends as a kind of unintentional suicide.