There are a lot of questions about Avengers: Endgame that went unanswered, and funnily enough, one is why Thanos sits at the end of the movie.
Since we saw Thanos make his first appearance at the end of The Avengers, fans have seen him do three things: search for the Infinity Stones, threaten people and sit. He even did all three in Guardians of the Galaxy, which I’ve got to say takes some skill. Finally, fans saw the Mad Titan take one last seat after his defeat in Endgame.
A few fans began to wonder why he took a seat (of all the things to focus on, this is definitely one of the weirder). There were several floating around, and while many make sense, a few are a bit… questionable.
Thanos is still an honourable man despite all his flaws and ridiculous logic. After spending thousands of years searching the galaxy for the Infinity Stones, torturing his children to ensure they were the best combatants alive, committing genocide several times and travelling back in time to fulfil his destiny, Thanos has got to be exhausted. At the end of a gruelling battle, he thought he could end it all. No longer would he be plagued by the thoughts of people suffering because they didn’t have enough resources. No longer would he have to fight for his ideologies. And he was right, just not in the way he was expecting.
After realising that all his efforts had been for nought, he doesn’t scream about the unfairness of it. He doesn’t taunt Tony in his last moments. Instead, he looks around as his army disintegrates into nothing, takes a seat on the war-torn battleground and accepts his defeat, content that he had done his best. It is a very calm and quiet end to a man who has brought so much destruction to the galaxy.

Cholan Raje on Quora suggested that Thanos might have had digestive problems, and that’s why we always see him sitting. Raje explained that he had bad gas, so he could easily pick up on the signs of someone else suffering the same fate, and Thanos was one of those poor sufferers.
They point out that he had two opportunities to kill Thor (on the Asgardian ship and during the battle of Wakanda), but he didn’t because he couldn’t let his enemies see him sit. “It would’ve conveyed the idea that they are superior to his might and power”. They further pointed out that it had probably taken him so long to react to Loki’s failure because he had been unable to control his gas.
They believe that every time Thanos is seen sitting, he’s quietly trying to deal with his gas. As his armies disappear around him, Thanos sits down, finally getting a moment’s peace to rid himself of the gas that consumes him. And in fading away, finally ascending to a world where he never has to deal with his gas again.
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Why do you think Thanos sits at the end of Avengers: Endgame?