I wanted to keep this in the tags but it was too long u_u...sooooo I agree with a lot of this but....
with a recent post I saw that put into words smthing I couldn't pinpoint; I'd like to bring this to the table: 4x16 and Dean telling Cas he "can't do it". And then Cas spending the rest of his life trying to keep the burden off Dean's shoulder despite the fact that Dean's main need changed from "needing out" to needing Cas by his side.
Cas started out by finding in Dean someone to whom he could relate. Who helped ground him in reality throughout s4 (which is how I read the "because of you I cared" from 15x18; Cas used to care from afar like how you care for ppl in a different country; for a cause vs caring for the people in your community; your neighbor; your friends or your family). Thing is Dean does get his own strays and it's something else Cas found they had in common (something Cas never got to explore for himself before). And Cas likes his strays; he's very fond of them. And yeah the more nurturing they are; the more fond he is but I think it's more that it's a bonus that reminds him of his favorite things about his human fam. Bobby and Dean are both gruff caretakers and while he knew Ellen very briefly it's something she showed too.
Meg's not a caretaker. She was like a spitting stray kitten but she was also the one most similar to Dean mostly because of their past with Alastair. But she's the one who attaches herself to him because she needs a mission and he's the one giving her one while not being the worst asshole to her. And she tries to make him react through quips and references ! Tell me she's not Dean coded. He becomes very fond of her ! Because when she's not torturing ppl she can be funny and likeable in a way he understands.
I know ppl love to ship them but the way Meg uses seduction as a way to throw ppl off is very similar to Dean (again) while Cas kissing her was based on just seeing a porno of a woman becoming distracted through kissing & sex and needing to distract Meg
Then Daphne...Well honestly there was little choice there. He had 0 idea who he was; what to do with his abilities; how the world worked and Daphne was there. Like his first time was with April that means his and Daphne's marriage was a celibate one so they seemed to care for e/o (which...they've spent a lot of time together for like 6 to 9 months) & she helped him and helped him find a purpose (healing others) but I don't know how much you can say he 'chose' this. It's very situational as she was literally the first person he met and seemingly the only one who cared enough to stick around
Aside: I honestly think a modern AU would have Cas as a Paramedic in huge parts bc of this. Like Cas is a tank but also a healer so my best picks for him were that or army medic (or both à la Eddie Diaz I swear 9-1-1 didn't exist when I first thought about it)
Then April is again different because he needed help and he was at her mercy. She was actively manipulating him and his owing her.
I personally think part of why he doesn't consider being taken advantage of is because of his pride bc it'd be admitting to being even more vulnerable than being human already made him. In a way sex was a way for him to do something for someone he owed not fully I don't think he'd have done it if he *felt* forced but the waters are muddy and it was rape even if we can nuance how it happened.
(that's how manipulation works)
NOW you also have Rowena who he becomes fond of. Why ? Because she's genuinely happy to see him. Rowena is not a caretaker *at all*. It's actually funny for him to be fond of someone who is Sam coded when he despised crowley & doesn't particularly like Sam. BUT he did pity crowley and he did get used to Sam the way you get used to & end up fond of your boyfriend's annoying pet. Which is how it started with Rowena. But also...At some point she ended up thinking he's cool and being flirty and while Crowley used his own feelings for self protection; Rowena also just genuinely enjoy seeing him. When they're on the phone with her she's happy to hear him; she can't see his reaction so she's not doing it to get a rise out of him (though she does enjoy the reaction she gets). Still, like Meg, Dean, or Crowley, Rowena does flirt and make quips.
Then I we also have the anchor lady who can go from fierce to caring (again who does it remind you of)
I know ppl like to consider them romantic but the closest to romantic in there as far as I see it would be the crush anchor lady or Rowena but crushes in general aren't necessarily romantic or sexual and while I see why ppl could see Cas as Bi bc of those ; to me he reads as AroAce (a horny one; but aroace nonetheless)
So tldr : I don't think he gravitates toward caretakers specifically so much as he recognizes their use for the tank he is and as a bonus they can remind him of the first people who helped him care on a more personal basis. He does enjoy flirty ppl who use quips to communicate though. I think it's part of why he's very fond of Claire.