I think the answer to your question is that he is referring to the natal chart as corresponding to the desires that have LEAD to the creation of the current life. So Pluto implies the kinds of desires the soul has been working out prior to this life, setting the stage for the current life. 

This doesn't mean these desire dynamics aren't operative in this life, of course it is. But in the Pluto book (which I assume you are referring to), his languaging is around identifying the past context leading to this life (Pluto through the sn...). So my guess is that is why.

Regarding the question "can a desire be fresh in this lifetime". It wouldn't be created out of no where - everything is birthed through experience as the soul then exhausts desires and generates new one's - an ongoing process. But the soul isn't going to pick a "new Pluto archetype" so to speak just to do something new. All new desire proceeds from the evolution of the existing soul desire context.

So we can absolutely generate all kinds of desires within a given life, but we are starting from the context of our existing soul dynamic and the desires that brought us into this life. Evolution of desire will be based on that.

thank you, Ari, that is clear. i was kind of imagining that in their previous lifetimes a soul might have completed their desire to self actualize professionally and socially, and in this lifetime they are moving on to a new desire to actualize spiritualy, something like that.