Thanos the Mad Titan. The Destroyer of Worlds. The Decimator and genocidal Universe balancer. Thanos... the misunderstood. The misrepresented. Thanos the chameleonic villain who is absolutely nothing like he was when he first appeared.
You've probably got a fairly well established idea of what Thanos is and what he's always been. He is the obsessive orphaned alien intent on bringing all of the Infinity Stones together because his planet's death instilled in him a fundamental urge to rebalance the Universe for its own good. Whatever it took, to misquote the Avengers' new mantra.
But Thanos wasn't that to begin with. His reason for seeking the Stones might not have even been what it would become in Avengers: Infinity War. Because plans change and the MCU's plans for Thanos changed as the character evolved. Remarkably, he did a great deal of that evolution in the shadows, his gestation period happening off-screen as the Avengers and the Guardians began to unpick how deep hid nefarious influence on their stories really went.
Despite the fact that he spent so much time off-screen in the early days of the MCU, The Infinity Saga is Thanos' story more than anything, and looking into the psychology of the character and how his motivations seemed to change and how he evolved as a character over the first decade of the franchise offers an intriguing parallel of where the MCU itself has evolved.
So let's start at the beginning, shall we?
6. His First Motivation - The Avengers
He might have shown up very briefly in the first Avengers movie, but there was still a fair bit of heft behind him and his presence looms over the film as the puppet master behind Loki's plan.
On paper, this Thanos is fairly one dimensional - and also not all that great at achieving his own agenda. What we know of him, through his interaction with The Other (and also through what that creature says about him second hand) is that he seeks universal dominion. The Other says as much to Thanos when talking about Loki's invasion:
"The Tesseract has awakened. It is on a little world, a human world. They would wield its power, but our ally knows its workings as they never will. He is ready to lead, and our force, our Chitauri, will follow. The world will be his, the universe will be yours. And the humans, what can they do, but burn?"
There's no suggestion there that Thanos was interested in fixing the universe through the Decimation and it would be curious if The Other wasn't aware of what he was looking to do. Think about it: when we eventually meet the Black Order, they are zealots to Thanos' cause. He is not so much a father to them as a religious leader, with Ebony Maw acting as his priest.
Crucially, they know what they're involved in, they believe in his cause. For The Other to have somehow been duped into thinking Thanos was simply interested in Universal dominion is improbable at best. The reality is that it wasn't his plan at this point to rebalance the universe. In actual fact, he didn't have a real plan.