#sam also has a pushy analytical style

[shallowseeker]

I think one of the things that initially attracted Dean to Cas was his manner of speaking.

Dean finds that he likes the straightforwardness and abruptness. Sure there was, "Are you allergic to straight answers, you son of a bitch?" in early days, but then there was also delightfully dry, one-word responses like this:

DEAN: Where've you been? CASTIEL: Jerusalem. DEAN: Oh, how was it? CASTIEL: Arid. CASTIEL sets the jar on the table. DEAN: What's that? CASTIEL: It's oil. It's very special. Very rare.

and

CASTIEL sits down. DEAN: Okay, so we trap Raphael with a nice vinaigrette? CASTIEL: No. DEAN: So this ritual of yours, when does it got to go down? CASTIEL: Sunrise. DEAN: Tell me something. You keep saying we're gonna trap this guy. Isn't that kinda like trapping a hurricane with a butterfly net? CASTIEL: No, it's harder. DEAN: Do we have any chance of surviving this? CASTIEL: You do. DEAN: So odds are you're a dead man tomorrow. CASTIEL: Yes.

Sure, Cas is kind of abrasive, but there's also very little natural subterfuge and artifice in his manner of speaking. He's not like John, passive-aggressive and wrapped with hidden barbed wire. He's not like Sam, overexplaining to the point of obfuscation. And Dean and Bobby are laden with pop culture sarcasm, emotional evasion, and other prickly defense mechanisms. (Dean loves Bobby, but his own style of communication exhausts him. He doesn’t love it when it’s coming from others.)

Cas generally says what he thinks, and he doesn't beat around the bush. And Dean finds Cas's speech pattern disarming...and really, really comforting, even when it's "rude."

It's why Dean's so thrown and hurt when Cas starts acting cagey and evasive in season 6. It's why the lying rips the rug out from underneath him.

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As for Cas, he's fascinated by Dean's manner of speaking.

Cas is incredibly inquisitive and curious by nature. (The first thing he does on arriving in the barn is to start rifling through books and spells without even making much eye contact with them.)

We can gather that Dean's inexplicably complicated speech patterns are something of a puzzle for Cas. He's always trying to read Dean's body language and put these strange references together with enough context clues to read Dean's meaning. Even more fascinating to Cas is that Dean's words and body language are often in extreme mismatch (especially when he’s posturing).

For a creature that knows so many languages, Cas is frustrated by this as much as he is charmed by it. Cas is enamored by Dean's sometimes volatile emotions and his ability to hold two conflicting viewpoints at the same time. (Hello, "Tender one moment. Tough the next.") Dean's speech reflects his cycling emotions and rapid-fire ability to take in and regurgitate information.

And Cas likes that.

He also likes learning the modern human quirks of language. In seasons 14 and 15, we see Cas practicing and learning human metaphors. He enjoys TV, and he really likes sharing this cultural exploration with Dean.