We know Dean loves pie. That's a given. We also know that getting that pie isn't so easy. Oh sure, he's scored before, like in season five's "Dead Men Don't Wear Plaid"...
But most of the time he's denied the pie. This one goes all the way back to season one in "Scarecrow," leading to the very infamous line, "I hope your apple pie is freaking worth it!" It doesn't help when Sam forgets the pie, like in season four's "Are You There God? It's Me Dean Winchester." Or when he can't get the pie because he's abducted by demons, like in season two's "All Hell Breaks Loose Part I." In season seven's "Girl Next Door," Sam even stooped so low as to think that cake was an adequate replacement for pie. As if!
My favorite denial came in season eight though, when Castiel is on a supply run for Dean and Sam in "Clip Show." He really wanted to make things up to Dean and the convenience store being out of pie didn't do much for his edgy mood. At least he scored a copy of Busty Asian Beauties!
You don't understand. I. Need. Pie.
At least this season, Mary bought Dean blueberry pie and he got to enjoy it. See, sometimes it just takes Mom to make it all better.