This is an emerging theme of Peanuts strips, where one character reads something in a book and reacts to it. Often these reactions are solitary, just a way to present a statement for a character react against.
Usually these statements are meant to be accepted uncritically. Not in this case however, here Snoopy's reactions directly disprove the book's declaration, which makes the strip into a commentary upon it.
As for whether dogs really can't reason, well, I doubt that severely. I've known a couple of very smart dogs in my time, one really quite freakishly so.
Charlie Brown's personality is still fairly mischievous at this point. He doesn't do this much more often. It seems like all the mischief in his soul disappears as it increases in the other characters, particularly Lucy. So maybe there's some kind of spirit transference going on here.
The text of ROASTED PEANUTS is copyright 2009-2011 by John Harris. No copyright is claimed over the comic strips, which are here under the principle of fair use. Strips presented for review purposes only. We love Peanuts a whole lot, and wouldn't dream of exploiting it. Please don't sue us; we're only trying to love. Thank you for reading this notice.