Because I can’t stop with thinking of stupid angel stuff theories/headcanons:
– The reason why Castiel doesn’t remember the darkness despite being there is because he’s actually the archangel Cassiel/Kaziel with memories and powers that God locked away after they defeated that great evil.
– Cassiel was very sad and had too much heart (archangel of solitude and tears) and God wanted to protect him. After Lucifer’s fall, most of Cassiel’s grace was taken in order to downsize him, along with memories of the darkness and his being an archangel.
– God erased Cassiel’s existence from the memory of every angel (including Lucifer, who already fell). He planted memories of the angel Castiel instead, who was a good soldier but not any “special” sort of angel. At least not yet.
– Although Castiel retained the problem of having “too much heart,” which was ingrained in his very existence. Hence why he eventually fell for the Winchesters and turned on heaven.
– In rereleasing the darkness, Sam and Dean have pissed off God, not only because he needs to fight this shit again, but also because with Michael, Lucifer, and Raphael out of commission, he needs all the help he can get. Thus, he needs to restore the rest of Cassiel’s grace and memories to Castiel, putting Cas back into fighting shape and archangel status, complete with badass armor and everything:
(This headcanon right now is underdeveloped but I have thought Cas was more than what he appears for a while now, and this whole darkness setup seems to be something that will make God and powerful angels both players again.)