Joker has been sitting there for a long time. Such a long time he couldn't really give you an estimate because he's been spending said-long time drinking steadily from his glass. It's some kind of alcohol Tali mixed up while being drunk herself, so that explains the burn, the weird banana taste and the incredible efficiency at making him plastered.
He's been sitting there a long time and everyone has stayed cleared of the two-metre radius around him. He's giving off a kind of edge no one has been willing to challenge yet. Good, he's been thinking periodically, whenever the alcohol is giving up some ground to his abused consciousness. Oblivion, dearest friend.
Joker is, indeed, drinking himself into oblivion because this has officially been dubbed 'Shepard's survival party' and he is not ready to make a party out of that particular event yet. See the part where he's the reason she died in the first place.
The worst thing is, he doesn't want to ruin all the fun for other people. He knows intimately what it means to see someone you thought had died come back, healthy and smiling, ready to take on the world. He absolutely understands why Garrus and Wrex are smiling like lunatics, why Ashley is teaching Tali to cheat at cards, laughing so hard she's choking on her drink. He gets it.
Joker had his five seconds to feel like he was crumbling inside from the sheer joy and relief, that moment when Shepard left to explore her brand new ship and he was left alone at the helm, crying so hard he had to bite on his cap to muffle the sounds and not worry anyone. But when that passed, all that's left is a crippling guilt so profound he feels like it's tainted every cell of his body.
Shepard was a beacon in his life, throughout all of their chase of Saren. Once he understood that she didn't give a flying fuck about his disease, that the only thing that mattered to her was how well he could fly their ship in and out of trouble, well. Suddenly, Shepard stopped being his Commander, his superior officier, and insteand morphed into that weird friend you adore more than you should, who you feel so lucky to be able to call yours.
On top of that, there had been the flirting. Maker, the flirting. The worst thing is, he's seen it done to everyone else on the ship. He's seen a Krogan blush, for fuck's sake. Tali was a mess every time Shepard got out of Engineering, a triumphant smile on her scarred face, only to dawn on Garrus like a vulture. He's listened to hours of banter that was more dirty proposals than soldiers bickering.
So when Shepard turned it onto him, he was kinda ready. Kinda being the important word here, because damn, when Shepard got onto it, she was relentless. He had stopped counting the times he hid underneath his cap, speechless, because she had just said something so salacious he pulled out some bullshit excuse to make an urgent checkup on the systems and try to rebuild his composure.
He failed every time, of course.
He watched Shepard flirt her way through the ship without ever taking anyone as a lover, despite literally everyone offering at one point or another, more subtle in some cases than others. And despite his best efforts, Joker had caught the bug as well. When they got to Ilos, he was well and truly done.
He thinks that Shepard even caught onto it at some point. When she was awarded her weight in medals for saving the Citadel, her crew by her side helping her carrying the crate of military rewards, she had brushed her hand against the small of his back and smiled when he shivered.
Then they got that mission to clear out the remaining geth outposts that Saren built, and they stumbled on the Collectors, who chewed them and spit them out on the other side bloody, destroyed, and a Commander short.
They never got to make anything of that flirting, of that heartsick feeling Joker had been carrying around, of that hope Shepard gave him at the ceremony. They never got to make the best of it, to figure out how to skirt around the regulation, stealing kisses behind crates and laughing with mischief, hugging and relieved whenever a mission ends without casulalties. Instead, what Joker got was a funeral with an empty urn that no one brought home because Shepard doesn't have any family left. An urn that got a place on the very selective Spectre monument in the Presidium, a towering stone where spaces are carved to put the beautiful ceramic and silver urns and where people left flowers every day.
Joker was left with guilt the side of the Normandy and an honourable discharg curtesy of the Alliance Navy.
And now? Now they want him to party, to celebrate Shepard's comeback, when he was the reason she was gone in the first place? They want him to forget what could have been, in the safety of her cabin on the SR1, on leave when they decided to take it, what could have been in the fantasies he painted in his mind during the months Shepard focused all of her flirting skills on her helpless pilot.
He's supposed to party and forget that he killed the woman he's in love with.
No matter what they think of him, Joker is not that strong of a man.