Though Thanos never specifies how he survived the destruction of his home planet Titan in Avengers: Infinity War, a reason does exist. The Mad Titan explains to the Avengers that his once lush and beautiful planet ended in tragedy, and some details that were ultimately left out of the film's final cut reveal how he managed to make it out alive.
In Avengers: Infinity War, during the battle on Titan in which various members of the Avengers and Guardians of the Galaxy face off against Thanos, it is revealed that Titan was ravaged by overpopulation and that its inhabitants eventually fell victim to extinction. While tragic, it ultimately inspired Thanos to seek out his trademark (albeit morbid) form of planetary survival: wiping out half of an entire population so that the remaining inhabitants will have a better chance at survival. As it so happens, this very mindset is what kept him alive when so many of his fellow Titans died out.
In Marvel's Avengers: Infinity War - The Art of the Movie, it's revealed that Thanos didn't just get lucky when he survived Titan's downfall, but instead survived on account of a proposal he offered in hopes of saving the planet. Some time before the destruction of Titan, Thanos suggested that wiping out half of Titan's population would offer the planet a rare opportunity for survival. However, due to this suggestion, Thanos was exiled. He had no other choice but to stand by as his people were wiped out and his planet was left in ruins.
While this motivation and backstory is obviously very different from the comic version of Thanos, it's very compelling in its own right and arguably fits more naturally into the MCU. In fact, in a strange way, it sheds some light onto Thanos' skewed ethical nature. As horrific as his plan to wipe out half the universe might be, it's not geared around some stereotypically villainous drive. Thanos' goal is - in his own dark and twisted way - peace; and the details of his tragic backstory at the very least lend some weight to justifying his actions.
Whether or not Avengers 4 includes any more backstory to Thanos, the tie-in novel Thanos: Titan Consumed will surely shed some additional light onto the skeletons in his closet. Considering the title of the book, as well as a plot synopsis that promises to reveal "the origins of the most formidable opponent" any of the Avengers have ever faced, it's likely that Marvel fans will be offered a deeper dive into Thanos, his home planet, and how he ultimately came to be known as the Mad Titan
Source: Marvel's Avengers: Infinity War - The Art of the Movie
- The Avengers 4 / Avengers: Endgame (2019) release date: Apr 26, 2019
- Spider-Man: Far From Home (2019) release date: Jul 05, 2019
- Captain Marvel (2019) release date: Mar 08, 2019