Castiel was moulting, and (as Gabriel had so eloquently described the process multiple times before he left Heaven) it freaking sucked. True, moulting was not deadly, nor was it harmful, or even that painful. It was just so inconvenient, what with the week or so of incessant feathers.
See, it was normal, even encouraged, for angels to take a vessel - to give up the status of "wavelength of celestial intent" in favor of humanity and sensation and emotion. And while angels were in vessels, they were required to manifest physical appendages protruding from the vessel's back. Angels had to have wings in some form or another at all times. But no sane angel walked around with their extra limbs on display for the world to see-first of all, because they'd likely be captured by curious humans (or demons) and second of all, because wings, whether physical or on their true forms, were the equivalent of human genitals. Valuable, vulnerable, and usually only displayed as an invitation to copulate. No one just walked (as much as a wavelength could walk) around with them out. Even around other angels, wings were only displayed as a supreme gesture of trust because they were sensitive. They were the weakest part of an angel, but they were also vital for flight and certain mating rituals, so exposing them to just anybody was the equivalent of exposing one's genitals in a room full of lumberjacks. With axes.
Now, there were two methods that angels used to hide their wings from general knowledge. Most angels simply hid their wings inside their vessels, slipping the atoms of the wings in the space in between the atoms of the vessel. (Atoms were, after all, mostly empty space. An atom the size of a football stadium would have a pea-sized collection of neutrons and protons in the center, with the electrons orbiting at the edge of the stadium, the size of pinheads. It was relatively easy for an angel to simply compress the space in the atoms and force more molecules into the vessel's back than was strictly possible. This contributed to the high body temperature of angels, for the molecules rubbed against each other and collided, causing friction and subsequently heat. It also made angels significantly heavier than humans, for they were denser.)
The problem with the practice of forcing wings into a small space were exactly what one would expect - wings cramped and got uncomfortable just like any other limb when forced to stay in one position for an extended period of time. The second method of hiding dealt with this, but it had its own problems. Sometimes, when they were sure there were alone, angels took their wings out of their vessel's body and simply made their wings invisible to humans and demons. Unfortunately, the wings were construed so that this invisibility only applied to humans and demons. Other angels, creatures that were not demons, and even cameras could effortlessly see past the facade. Wings were... odd. Some angelic powers affected them profoundly, and some had no effect at all. (Not that any angel was inherently a "him" or a "her." What a silly language English was, with no extremely commonly used gender-neutral singular pronouns.)
But there was a time where all angels were required to have their wings out - biannual molting. The natural, restorative process of replacing all of the feathers on an angel's wing. Indeed, Castiel's wings were getting rather ragged. The feathers had dulled and the fronds had separated and ripped, and Castiel was grateful that his molting was happening when it did - the Winchesters seemed to have just finished a case and were in the process of looking for a new one, which could take upwards of a week or so. Castiel was grateful, he really was, but he was also supremely annoyed because of the relentless feathers.
Oh, Dad, the feathers.
Feathers falling out, feathers growing in, feathers on him, feathers around him, feathers coating every surface of the abandoned shack he had taken up residence in until all he could see was black. And his wings itched. They burned like fire, physical equivalents of nerves sending violent signals to the physical equivalent of his brain. And it was awful, because Castiel didn't usually feel. Anything. No physical sensation from his vessel whatsoever-and that was good, because pain in his vessel, he could deal with. Anything solely confined to the human he was occupying and the physical plane was bearable. The vessel served as a buffer of sorts between angel and sensation - if needed, Castiel could force himself to not feel any of the pain.
But his itch was not only on the physical plane, nor did it just affect his vessel, for the wings were entirely Castiel. They were the only things on the earth that were truly his. And it wasn't like Castiel had never molted before - he had, millions of times. But that was when he did not have a physical sensation to torment him. And then, he had his fellow angels to groom him and get rid of the old feathers.
No such luck now. He was alone on the earth, wings itching like he had never felt before in his long life and going through his first physical time of weakness. His very Grace almost vibrated, trying to find something, anything to rub against. Anything to take the edge off.
Which didn't exactly work, for there were no such things as wavelengths of scratching post intent.
And Castiel couldn't groom himself, because of the sheer size of his wings. When in Heaven, his siblings had always teased him about his larger-than-normal wings, but now he cursed them even more vehemently. He simply could not get them into a position where he could reach the entire expanse of the wing to get the old feathers off. He knew this, but he kept trying to find that elusive way to itch his scratch.
Three days passed this way. Three days of constant struggle, three days of wrestling at a problem to which there was no solution. Three days, six hours, twenty-seven minutes, and 8.2 seconds after his first feather detached itself from his left wing, Dean called.
Awesome.