The Marvel Cinematic Universe has yet to explore this yet, since they havent really explored Thanos in any great detail at all. As far as the cinema-going superhero audience knows, hes just a purple man whos come to resemble the older kid from The Goonies more and more, a mad tyrant who sits on a chair and lets others do his dirty work. In the comics, meanwhile, his quest for the Infinity Gems very much has a purpose, and he actually bothers to go tracking them down himself. Though he does find a lot of time to sit on that floating space throne as well. When he gets a hold of all power over reality, Thanos has but one goal: to destroy it. Destroying everything is a weak villain motivation, usually, a sign the writer hasnt though much about the character except that theyre bad uns. In the case of Thanos, though, he wants to kill everyone in the known universe to impress Death, his one true love. He is head-over-heels for the Grim Reaper, and that much destruction is like foreplay.