I headcanon that Mako actually has a kinda goofy sense of humor deep down, but he's spent his whole life having to be the Serious and Grown Up one in every situation so he never gets to be silly. And now he's been the serious one so long even in casual settings everyone has already mentally assigned him the role of stick in the mud/serious guy.
I also headcanon Mako as extremely literal at times and doesn't always think things other people think are funny are actually funny. Because he doesn't always share the same sense of humor as the people around him, or takes longer to process a joke, people write him off as humorless when he isn't.
But! Wu has no idea who he is when they meet. And Wu meets him without the context of Team Avatar or even Bolin. So while Mako is very stiff and professional in public with him (back to assigning himself the role of 'serious guy' to Wu's goofy shenanigans) when it's just the two of them their relationship is a blank slate. Mako can be a little silly, can joke around with Wu, because Wu *doesn't have any expectations of who his is/what 'role' he's supposed to fulfill in a group setting*. Mako can be whoever the hell he wants with Wu.
They have little in-jokes. The first time Wu makes Mako laugh absolutely stuns him in the best way and Wu spends the rest of the day telling increasingly absurd jokes and stories just to make Mako smile again. Wu turns getting Mako to break his professional blank face and smile in public into his own personal game. They laugh about all the crazy rich people stuff they see at the parties Wu goes to.
I firmly believe that, at least in private, Mako lets himself be a little silly with Wu.
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