Castiel knows that Derek understood the choices he had to make in that other place. Castiel also knows Derek's understanding was likely motivated by terrible self esteem and guilt but regardless...

It is the least he can do to contribute a pie and some liquor considering he is certain Dean is likely to make off with the majority of both.

"Hello, Derek. Do you want to exchange declarations of thankfulness, as is custom?" he says, clutching a pecan pie.