I think that this becomes one of the central issues in the novel.  The idea of "it" and its pursuit is something that drives both Sal and Dean.  The moment and living for it represents a critical element of the primary motivation of Sal and Dean.  Both of them seek to live for this "moment," something that is fundamentally different than the conventional notion of American society in the 1950s.  In contrast to planning and living a life of socially dictated normative behavior, the pursuit of "it" is the reveling in the moment, the instant in which life's meaning is evident.  It is part of the journey in which the destination is not as important as much as the freedom to live out the voyage.  This is "it" for both men.  Sal and Dean and the friendship they share is a part of this engaging in the moment.  This pursuit defines much of their happiness for it is an experience of an instant, something that cannot be predicted, fore-casted, or even recreated.  It simply happens and it is for this reason that "it" is so loved by both of them throughout the narrative, forming both the basis of their friendship as well as the essence of the novel's legacy.