hard to say why it makes me want to scream and puke at the same time, because i think there’s a couple of reasons (for me, at least) and i won’t even deny that they’re somewhat rooted in headcanon.
first of all, can you say: “pronunciation boner”? i just love the way dean/jensen says it compared to most other characters. some people tend to skim over the vowels in the “-iel” (cast-yell) or merge the t with the i and you get a weird almost ch sound (cas-chiel or ugh cas-chyell. i think naomi did this a couple of times and it near drove me up the wall) and in many cases it sounds more like two syllables than three. but dean is careful with every syllable, all three (ugggh listen to him say it right after pam says it (flawlessly) — it’s beautiful). so he just doesn’t say “castiel” enough to have developed shortcuts, or he’s just subconsciously reverent enough to give the word his full attention.
secondly, it reminds us of all the times dean doesn’t call him castiel. as you probably know, the suffix “-el” essentially means “of god”, so there’s the divine element to castiel’s full name that isn’t present in the nickname “cas”. dean, bless his heart, doesn’t always respect that part of castiel because of his general frustration with all things related to god. and despite castiel’s ongoing disenchantment with heaven, it remains an integral part of his identity, but not one that dean always respects. so when dean calls him “castiel”, it’s almost like he’s affirming that part of castiel, acknowledging his divinity as an important part of his being, respecting that part of castiel even though he has his own personal beef with god. castiel is of god, but he’s also dean’s best friend.
and ugugghhhh can you just imagine these two horizontal with dean riding a fully wrecked cas and whimpering nonsense — endearments, curses, and “cas” — over and over until they’re both standing on the edge and dean leans forward as far as he can go and cas closes the rest of the distance to meet him and dean’s kisses slide off cas’ mouth and towards his ear where he hisses hot into the sweaty skin there “castiel” and that’s it, that’s all folks, because they’re both completely fucking done.
doesn’t it make you wanna just—