As in life, everyone has different ideas about the nature of friendship in The Picture of Dorian Gray. To some, it entails great things like devotion, admiration, and loyalty – but, to others, it means a kind of relationship of mutual interest and temporary companionship. To others still, "friend" might as well be totally replaced with "frenemy." However you swing it, friendship is an important recurring theme in Wilde's novel, and its different definitions cause a lot of conflict.
There is no vision of true, egalitarian friendship in The Picture of Dorian Gray; all of the relationships are tinged by other elements, such as idolatry, jealousy, and resentment.
Wilde's characters are fundamentally friendless and alone, and even if they don't recognize that this is a problem, it makes true happiness impossible for any of them.