This fucking moment right here!!!! This is the moment when Brian finally understands that him and Justin are endgame. That Justin is it for him. That no one ever has nor ever will be a more perfect match for him than Justin.
Because only Sunshine can take care of him while also calling him "a mother fucking piece of shit". And mean it. Mean both things at the same time: you're a fucking asshole and I still choose to love you. Only Justin can see through him, and accept him and love him for who he is, while also challenging him and rising to his level.
And I think this is the key difference between how Justin relates to Brian vs how other characters do. Even the other characters who have some romantic feelings for Brian, like Michael and (arguably) Linds. To them, "loving and accepting Brian for who he is" means "playing by his rules". They let Brian dictate the dynamic of their relationship to him, and they mold their expectations to fit that.
But Justin doesn't do that.
Yes, he sees and understands and accepts who Brian is, and he loves him for it. But he's also the first and only character to ever challenge Brian's bullshit. Not just call him on it, like we see many characters do many times (Debbie, Michael, Linds...). But actually challenge it.
Everyone else just takes Brian's rules at face value. Some, like Mel, out right reject Brian's rules. Others, like Linds and Michael, accept them (either passively or begrudgingly) in order to keep Brian close. But they all still accept that "that's just who Brian is" and that's how things are with him.
But where everyone else takes Brian's rules as immobile, Justin is the first and only one to say, "ok, we'll do things your way, BUT we're also doing them my way". He bring his own set of rules to the table. He negotiates with Brian. And he does so multiple times throughout the show: when he tops Brian, when he asks for the no-kissing-others rule, when he convinces Brian to let him intern at Vanguard... And in this moment as well.
This moment is Justin telling Brian yet again, "Ok, we'll do things your way (angry and cursing at each other and with you trying to keep me at arms length bc it scares the shit out of you to be this vulnerable), BUT we'll also do them MY way (you'll let me in and I'll take care of you, because I fucking love you, you fucking asshole)."
And I think this is the moment it finally sinks in for Brian: that Justin loves him in a way that no one else has before. In a way that probably nobody else ever will: understating and respecting him, but also challenging him. This is the moment Brian realizes that Justin loves him exactly the way that he needs to be loved.
And I think that's beautiful.
