It's apparent from the movie that Arthur loves paradoxes. And maybe that's because he is sort of a paradox himself, he's full of little contradictions and I doubt he even realizes it most of the time (but Eames does and it's what made the pointman so interesting at the beginning).
Arthur is analytical and meticulous, he likes precision and details, and makes plans and contingencies; and yet he is so skillful at improvising, and even enjoys it in a way, outsmarting sudden unexpected problems, at least when they're not risking Limbo (so much for having no imagination). But even more interesting is how he presents himself to others as calm and detached and yet he cares so much just about everything.
And more interestingly, while his emotions are always clearly visible on his face (thank you JGL for your expressive afce), he keeps himself on a tight leash, rarely allowing himself to actually act upon it, at least not without passing through some kind of filter of rationality first. That is, unless they're in the direst situations with adrenaline and stress running high.
And unless the person at the other end of the exchange is Eames. Because Arthur too acts different than usual around him.
Arthur, who is always so poised and collected, or at least strives to be, allows himself to engage in the banter Eames initiate, and to just react instinctively to his jabs. And even if most of the time that reaction is based in mild annoyance and competitiveness, it still feels like friendship and trust. Because for once Arthur is letting himself really feel and inhabit it his emotions, something he does with no one else. And then shit hits the fan and we really see just how much Arthur really cares for Eames when he flips in the taxi scene and when he helps him with the PASIV just to exchange a few soft reassuring words that may or may not be their last ones to each other; and how much he relies on Eames, even subconsciously, when he relaxes after the darling comment.
He feels safe enough with Eames to lower his guard and loosen himself up, to show even the most imperfect, less polished, 'ugliest' side of himself, sure in the knowledge Eames doesn't really see him as just a pointman or a mentor or whatever, but as a person and a friend. And it's liberating.
