#Sam on the other hand was more a lightning rod of negative interaction in a more active way

[suncaptor]

While I do think in general Sam & Dean both know each other very well because you know. they're siblings who've known each other their whole lives often to a codependent degree. I think that in terms of where they lack knowledge Sam just has like less raw details of Dean from childhood by nature of Dean trying not to be as vulnerable and also just Dean being older. Like he didn't even know Dean carried him out of the fire for example, or that monsters were real, but the ways these things impacted Dean he was noticing and working with for years.

Meanwhile for Dean I think the issue comes down to assuming he understands Sam and pigeonholing him and then – when he's too emotional – missing him. Which I personally find more significant in terms of feeling known myself, but I do think when Dean's more stable this is far less true. And Dean has Sam memorised far more than necessary for a healthy close relationship.

But these things become somewhat moot considering Dean literally does not know what happened to Sam in Hell & Sam will always see far too much the best in Dean while also assuming he himself deserves the worst. So ironically they see each other clearly worse as we see them on into the show spending all their time together.