Hrumdedum I wrote this a few days ago on the train back to London and depending on what way I look at it I like it and I don't.
He's furious that Sasori's dead. Or rather, he's upset that Sasori's dead and he's furious that he cares. At the time he shrugged it off like it was nothing and forgot about it, because it was easier to let it take a few days to sink in that Sasori was actually dead and gone. Forever.
Deidara has never been good at dealing with the concept of forever. He always used to tell Sasori that nothing was ultimately anything – nothing could be for always, nothing could remain the same, nothing was definite, nothing lasted, that you could never promise anything, that there was nothing that was completely one thing or another, and that there was no such thing as forever.
Deidara's always gone back to Iwagakure now and then, to walk down the familiar streets that are still standing and feel the familiar atmosphere and bomb them. The beauty of every explosion is fleeting, but he'll always set another one off soon enough.
Everything is temporary, but the afterwards has always been temporary, too.
Except Sasori's not coming back.
Being alone didn't last too long, because now he's partnered with Tobi and Tobi is eager to spend time with him, but Sasori is gone forever.
Deidara is angry beyond measure that this is how he had to be proved wrong.