You can't erase the fact that your listening today has matured and benifitted by forty some odd years of musical evolution (whether you were here and listening all that time or not) since these record had their inception.
The hard part of understanding these landmark albums can be enhanced if one did not experience them during their time of introduction, in context to what music was currently in rotation, going back and listening to the billboard hits of that era and realizing the music and social conscience of that moment. You may well be surprised just how revolutionary the concepts and how dwarfing the musicianship and production skills in juxtaposition. But sitting here in 2009 saying "I don't get it" may just be not understanding the context.
With the benifit of Pet Sounds and SPLHCB in retrospect, why does it still seem to be so illusive, creating such a work out of thin air?
Happy Listening!