Since the Avengers and their allies have continued to protect the world from threats too big for any 1 hero to handle, a new threat has emerged from the shadows: Thanos. A despot of infamy is to collect all six Infinity Stones , artifacts of power, and use them to inflict his will that is jagged reality on all. All of the Avengers have fought for has led up to this moment - the fate of Earth and existence itself has never been more uncertain.
After six decades of buildup, the Marvel Cinematic Universe needed to deliver on the promise of Thanos. Marvel Studios teased him and over multiple films, so the pressure was on for Josh Brolin to deliver a performance. His fascination in rebalancing the universe comes after seeing his homeworld of Titan destroyed due to him and overpopulation not being permitted to stop it. By the time Infinity War comes about, he's amassing the Infinity Stones so that he prove his theory right and can do it himself. He was finally victorious, devastating MCU heroes and fans alike (except for Bradley Cooper) at the procedure, which is why Thanos is grinning at Infinity War's final shot.