Spoilers for Avengers: Infinity War ahead.
“There was no other way,” Doctor Strange wearily explained to Tony Stark just before disintegrating into ashes. Shortly before then, he had used the Time Stone to see over 14 million possible outcomes for their fight with that lying liar Thanos, learning they could win the fight in “only one” scenario. He then willingly gave the Eye of Agamotto (which holds the Time Stone) to Thanos to save Tony’s life. This has sparked plenty of fan theories already, including a convincing one about Captain Marvel and now a plausible theory that ties together multiple little details in Infinity War you may not have noticed.
When Thanos’ henchman Ebony Maw (the Voldemort-looking wizard) tries to snatch the Eye of Agamotto from Doctor Strange’s neck, it burns his hand: