@inception30daychallenge Day 14:
Not my headcanon originally, but I absolutely believe that Arthur is protecting Cobb as a promise to Mal before she died.
ARIADNE What was she like in real life?
ARTHUR (quiet) She was lovely.
It just hits me every time. Between that, Mal knowing Arthur's artistic taste and Cobb asking Arthur to send his regards to whoever Arthur's visiting in the States (presumably Arthur's family, which suggests he's met them) - I think there's a deep relationship with the Cobbs as a unit than just "guy we work with".
But nevertheless, Cobb's still "Cobb" and not "Dom" to Arthur. They have a comfortable rapport (Arthur getting him back on track after Cobb starts talking about intestinal worms, Cobb joking about him getting shot in the leg) and work well together. But at the same time, after Saito is shot, the way Cobb turns on Arthur and immediately blames him seems really... coworker-y, rather than between two friends working to get Cobb back to his family. Which has always struck me as really weird, the way Cobb immediately cracks that relationship with Arthur apart.
Meanwhile, Arthur only starts getting emotional and personal once the Limbo thing comes up, then immediately brings up Cobb's time there and Mal:
ARTHUR ...anyone sharing the dream who's been trapped [in Limbo] before. Which, in our case, is just you.
ARTHUR "Done it before." What, with Mal? Because that worked so good?
But I don't think Mal & Arthur were ever romantic, because Cobb would be way way weirder about Arthur if it was. Hell, I'd even accept it as evidence towards Arthur being gay, because I think Cobb's in a place where he'd be extremely weird about a man who is attracted to women having had a close emotional relationship with his wife (again, she knows what the inside of his mind looks like!), even if that relationship was entirely platonic. So why else would it be safe for Arthur to say she was "lovely"? Plus Mal brings up Arthur's taste in art and interior design to Cobb twice. It does make the Ariadne kiss weirder, but we're already used to explaining that one away...
Bonus: due to this level of familiarity with the Cobbs, Arthur is absolutely on "Uncle Arthur" terms with Philippa and James.
help i dont actually know what day it is anymore