He knows he's not an outsider anymore. He knows that Sam, Kensi, Hetty, Eric, and Nell have all accepted him into their little NCIS family. Callen, he wasn't really sure about—truth be told he's still not sure about him. Sure, Callen was just as gung ho about finding who shot him as the rest of them, and sure, he jokes around with him, but Marty can't help but feel as if there's something missing. Something that everyone else has with Callen that he doesn't. For the life of him he can't figure out what it is or why it bothers him so much.
He knows, especially after this whole being shot incident, that Callen has his back just as readily as he has Sam's, but Callen still seems…stiff around Marty. He guesses that Callen is just being protective of Kensi. It's hard not to be…and, yeah, it may be sexist, but Marty gets it. Kensi's their girl, and no matter how unbelievably capable she is, guys like Callen (The good guys…the ones who choose to devote themselves to protecting people.) will always pay extra special attention to the safety and security of the women they work with. Call it instinct. And Marty, well, Marty doesn't have the same training and experience that Callen, Sam, and Kensi do. He's the weak link, and he knows it. So, yeah, Marty thinks that might be it.
But then he thinks that, no, that can't be it because if Callen really didn't trust him to have Kensi's six, Marty'd never have made it on the team. Hetty may be the final word on who's in, but she pays real close attention to how her team is doing. If Hetty didn't think Callen trusted him, or wouldn't trust him, she'd never have brought him on. She takes damn good care of them.
Sometimes Marty thinks it's that Callen just doesn't have a sense of humor. Like, maybe Marty's jokes annoy him, but that idea's quickly dismissed. Marty refuses to believe anyone could be permanently put off by his sense of humor. He's a funny guy.
He works hard every single day to be that funny guy because, really, funny-joker-Marty is so much more likable and reachable, than the broken, abandoned kid whose father thought he was a waste of space, and whose mother spent half her time drunk and screaming, and the other half drunk and passed out.
And maybe that's it. Callen hasn't accepted him because he sees that there's more to Marty than a good cop who likes to joke around. Maybe he recognizes that Marty's a kindred spirit. Someone society forgot. Someone who dragged himself out of a miserable, lonely existence and created a life for himself. Maybe Callen sees that, and maybe it pisses him off that Marty doesn't own that achievement the same way Callen does. Because Marty can see, clear as day, that, no matter what lies Callen weaves for his job, he's an honest man. He values honesty above most everything else because it's something he's been so rarely given. And the lie that is Marty Deeks probably makes his skin crawl.
So, yeah, everyone else, they've accepted him—he's family; but Callen, Marty's not holding out any hope that Callen's gonna let him become family anytime soon. Marty's comfortable with the lie—comforted by it. G's just gonna have to deal with that.