A Plan 20 Years In The Making And This Dude Be Rocking It Up - Tumblr Posts
Why is Law more stressed, more serious and often grumpy and annoyed in Wano? Thought he would be relieved after Luffy defeated Deflamingo 🤔he couldn't even relax at the banquet 😑
Broody Law is actually my favourite Law, which means I enjoyed him so much in Wano ❤ But why is he delivering behaving like that, indeed? Let's summarize the Wano experience from his side, shall we?
his alliance with Strawhats gets sidetracked because Luffy went to Big Mom to retrieve Sanji. He barely avoided a conflict (not really)
he's stuck at Wano, worrying his ass off for Luffy, hoping he's not left with half of Strawhats crew to take care for forever now
everyone is dying of hunger and getting sick in Wano, because they can't stop drinking water from the polluted river
Luffy finally arrives, Bepo gets sick, Luffy in the meantime does The Worst Thing Possible and Law's efforts to patch up the situation do not work out
Luffy snaps and runs off to fight Kaido
Kaido bombs away Strawhats and Hearts crews while Luffy and Law are already near town and can't do shit about it
he fails to stop Luffy from fighting Kaido
he fails to stop Kaido from putting Luffy in prison
freaking Hawkins getting in his way
crews turn out to be fine, but freaking Kinemon soon leaves to find remaining Akazaya samurais. Now the whole raid plan and making sure it's not exposed is Law's job. On top of that, he needs to take care not only of his Heart Pirates, but also Strawhats, because their captain is in jail. Two crews, a raid plan and the weight of 20 years of revenge is all hanging on Law's neck.
freaking Germa (he's a Sora follower, ok)
he almost throws it all away when Hearts get put in prison
he's got accussed that his crew is crap and he should just kill them off, all that coming from his very own allies he's been taking care of for last couple of weeks!
freaking Hawkins again
getting tortured (just another monday morning amirite)
finding traitor and making sure the raid doesn't fail in the process. Is Luffy finally out of that prison, what is he doing there? Or maybe, that's for the best, he can't make the situation even worse...
Luffy arrives late to the raid and late to the rooftop battle
Luffy brought freaking Eustass Kid to the raid
Luffy is ordering him around in front of freaking Eustass Kid
Luffy tells him to get along with freaking Eustass Kid
Zoro-ya almost died on his watch
Kurashi-ya (literally) caught him in a middle of an embarrassing blunder
Eustass Kid is behaving like a newborn baby and Law has to babysit him
LUFFY BROUGHT FREAKING BIG MOM TOGETHER WITH HIM
now it's Law's job to fight Big Mom. He didn't sign up for this!
Luffy nearly drowned in the sea while fighting Kaido
Luffy lost and died while fighting Kaido
Luffy got revived and won against Kaido
Law had to make bigass room, bigger than anything he made in Dressrosa, probably cutting down on his lifespan in the process
Law has to take care of stuff no one cares about after the raid succeeds, which is taking care of the ships, searching for the poneglyphs (no one but him cared for that in the middle of the raid!) and probably patching many, many people up
he ends up stranded with Robin in some moldy underground chamber for hours, while being seriously exhausted, sleep deprived and in the middle of recovery from his injuries
he doesn't join in the banquet out of spite (no one told him about Jimbei joining the Strawhats. Is Law a joke to them?!), besides just in case a certain Luffy would assault him there, make him go all night long eating and playing around and celebrating, all the while probably bugging him to become a 100% part of the Strawhat crew or at least extend their alliance to together forever status.
And that's just the major things listed! I'm sure I missed some details here and there.
Overall, I think he was stressed and exhausted, because he was carrying a lot of responsibility (and the stakes were extremely high!), at certain points almost the whole raid plan's success was depending solely on him, while others merrily ignored the dangers. He also had two crews to take care of for the whole arc, constantly worried himself over Luffy, got pissed at Eustass Kid and had to cooperate with him (he hates that dude!), and at the end of it his world got almost turned upside down in worst possible way (Luffy dying and raid failing as the result, and the vision of all of them dying flashed before his eyes).
As Bepo reveals to us in the midst of Winner Island escape, Law wants to stay behind with his crew even if it kills him and needs to be reassured that no one is dying there. That suggests Law doesn't deal well with people close to him dying, which shouldn't surprise us after what happened in Flevance and later also to Cora-san. I'm sure Wano hit Law harder than we expect, because there he was, finally believing in people again (Luffy), only for Luffy to die on him as well. Old issues must have resurfaced and his fear of losing people worsened as the result.
I think we should cut him some slack, shouldn't we? Wano was definitely not a walk in the park for Law, especially psychologically-wise. Though to be fair, he could relax and laze around a bit, we see it happen, right before Strawhat Luffy waltzes into the country. That was the last time we saw him actually calm and resting, so I guess that alone gives us another answer to your question as well ;)
And then his alliance ended, and we know Law doesn't like dramatic things, which also means overly emotional goodbyes. It was probably pretty difficult for him to part ways and maybe he thought it would be easier if he avoided Luffy for most of the time after the raid finished. And then Chopper went and almost blew all his efforts up with his warm send-off! Law owes a lot to Strawhats and his debt is only getting bigger, but it won't change his withholding personality.
Did Law find peace after Mingo was defeated? Not exactly, because I don't believe Doflamingo was Law's end goal. Just take a look at his reaction when Luffy finally wins:

Everyone is crying and celebrating, but Law's face looks like this instead. Does this look like a face of a person who finally got rid of his demons and is set free? What about Flevance, are we forgetting that? Law has a lot on his plate, Mingo was simply one of the obstacles in his way (but an important one!). If you want to know more about what I think about Law's end goals, I wrote about it here: https://www.tumblr.com/l-in-the-light/761556630027616256 (the topic starts somewhere around the second picture). That's why I don't think he is at peace or that his revenge is over and we shouldn't expect him to completely "move on" or reach perfect closure just because Mingo is now out of the picture.
There's one exception though. I do believe Law is showing his emotions more openly after Dressrosa. And I think it's actually a sign of him healing and partially moving on.

Because just look at this. Law was always annoyed with his beloved Cora-san as well, and yet we never doubt he loved him dearly. In Wano Law is acting the same way towards the Strawhats, openly showing when he's annoyed and angry. That's how he shows affection and worry. Compare it to Punk Hazard, when he's always holding back, stoic and cold, keeping his comments and reactions to bare minimum. Dressrosa changed everything in Law's emotional department. And the result is that he did open up to his allies.
Also I feel like it's my duty to remind the world about this, often overlooked, fact: so far Law celebrated in the feast exactly one time, in Dressrosa. He didn't join in on Punk Hazard, Zou or Wano. So it's not really a "Wano thing", but more like Law almost always choosing other things over feasting ;) personally, I think it's because of that "30 minutes on festival" rule he had in his household as a child. He's probably not used to just playing around for hours. Besides, we should never underestimate Law's pettiness. Not joining the celebration might have been as well just a petty payback of his, for every offense that happened to him in Wano, lol.
I'm not sure if you liked my answer, anon, but I hope you at least smiled a bit when I tried to mimick Law's grumpiness in this post :D