Thanos' original motivation for the decimation was to court Lady Death, but that was changed in Thor film when the Infinity Gauntlet was spotted in Odin's vault.
However, his first proper physical appearance was in the post-credits scene of Loki's defeat during the Battle of New York, the Other explained reported the God of Mischief's failure and warned his master, saying that to challenge the Avengers is "to court death." Instead of being dissuaded, the Mad Titan seemed to welcome the idea, breaking in a knowing grin. This heavily implied that just like in the comic books, the idea thrilled him as Thanos was actually obsessed with the metaphysical representation of death. Since then, the purple alien had appeared in various ways until his official arrival in Infinity War, but suddenly, he had a different motivation.
As shared by Gamora, Thanos was instead driven by bringing balance to the universe. While he had been slowly executing his plan by going through planets one-by-one, learning about the power of the Infinity Stones and how it fast-track his mission convinced him that it's better to collect the elemental crystals instead. Later in Infinity War, he justified his goal by using what happened to Titan as a supposed cautionary tale for what the galaxy can be. It's a more dynamic and compelling reason for his crusade, but aside from that, the filmmakers simply explained to Vulture that introducing Lady Death would actually be problematic for them. Co-director Anthony Russo said that since Infinity War was a culminating film, incorporating an essentially new character would make the film's story so much more complicated. “That would also be creating another level, another plane of existence, that the MCU possibly hasn’t gotten to yet,” co-writer Stephen McFeely added.
It's essentially the same storytelling principle that Marvel Studios used to explain why Adam Warlock isn't in Infinity War and Captain Marvel wasn't the one to ultimately end Thanos. Both Infinity War and Endgame were culminating films; they harkened a lot of elements from what was already set up and not introduce new concepts that would significantly change the trajectory of the story. Granted that Infinity War did reveal fresh information about where the Soul Stone was and the existence of Nidavellir, much of it had been previously known. There were debates about whether or not Marvel Studios should've tackled Thanos' backstory in a separate movie before pitting him against the Avengers. So one can only imagine how much more criticism the flick would've gotten if it shoehorned Lady Death in his arc.
Overall, Thanos' MCU motivation worked better than just wanting to court death. It allowed Avengers: Infinity War to incorporate his relationship with Gamora whose planet was manually massacred by her adoptive father. ittedly, the Mad Titan's plan was full of loopholes, but it's so much easier to sell the idea to viewers rather compared to having him be hopelessly in love with Lady Death.
