Thanos set out to wipe half of the universe's population out of existence in Avengers: Infinity War and ended the movie "totally satisfied" with himself. The movie put Thanos at the forefront of its story, making him the main character internally, and resulted in him achieving his goal with the snap of his fingers.
After six years of buildup, the Marvel Cinematic Universe had to deliver on the promise of Thanos. Marvel Studios teased him for years and across multiple films, so the pressure was on for Josh Brolin to deliver a memorable performance. His interest in rebalancing the universe comes after seeing his homeworld of Titan destroyed due to overpopulation and him not being allowed to stop it. By the time Infinity War comes around, he's gathering the Infinity Stones so he can do it himself and prove his theory right. He was ultimately victorious, devastating MCU heroes and fans alike (except for Bradley Cooper) in the process, which is why Thanos is smiling in Infinity War's final shot.
A new quote from Brolin is included in The Art of Avengers: Infinity War where he talks about the motivations of Thanos. He isn't a natural killer and doesn't even kill the Avengers in Infinity War unless it is absolutely necessary. He is doing this because it is what he believes is the only way for the universe to continue, so when the snap has its desired effect, Thanos feels nothing but satisfaction.
"He's trying to fix the universe. He's trying to balance the cosmos, and by doing that he's having to destroy a lot of things. So there's the warrior Thanos, and then there's the Thanos that you see in the hut. I personally like the hut Thanos because it's much more real. He's perfectly and totally satisfied having accomplished what he set out to do."
This is conveyed through Thanos' smirk in Infinity War. At this point, the permanently damaged Thanos is ready to sit back and look upon the new universe he has made. After balancing the population of individual planets in the past, he knows that this will work. But, if he is truly satisfied, then it does raise the question about his role in Avengers 4.
Thanos' motivations for Infinity War were abundantly clear from very early on, but his role in Avengers 4 is one of the many secrets that still remain. It is confirmed that Thanos will return and Brolin's teased some of the scenes he shot for the next film. Concept art for the movie even revealed a brand new sword and his return to the battle armor. So why does Thanos suit back up and fight again? Because someone is trying to undo his accomplishment?
Whatever the actual plot of Avengers 4 is, it is currently expected that it will somehow revolve around the surviving heroes' efforts to reverse the snap and bring everyone who was dusted back to life. This could be done through time travel, alternate dimensions, or even a combination of both. But, the second Thanos figures out that the MCU's heroes are trying to reverse what he's done, his satisfaction will fade and he'll need to protect the future he's created. So even though he's currently satisfied thanks to Avengers: Infinity War's ending, he likely won't be able to stay that way for much longer.
Source: The Art of Avengers: Infinity War
- Captain Marvel (2019) release date: Mar 08, 2019
- The Avengers 4 / Avengers: Endgame (2019) release date: Apr 26, 2019
- Spider-Man: Far From Home (2019) release date: Jul 02, 2019