Warning: contains spoilers for Eternals #12!
Marvel's Eternals recently revealed the truth of Mad Titan is obsessed with Death, and has been since childhood. Since his earliest days, Thanos has been stalked by Lady Death - the sentient expression of entropy in the Marvel Universe. Thanos has professed his love to Death many times, killing billions in her name, and yet it remains a mystery exactly why he's so obsessed, especially as he initially resisted the dark god's siren call.
In recent issues of Eternals, it's been revealed that Thanos was the result of an experiment to create new Eternal life. The Eternals are immortal beings created by the godlike Celestials to safeguard humanity, and operate more like parts in a machine than individuals with free will. Each of the exactly one hundred Eternals was created to be part of a larger machine, with predetermined personalities and relationships. The Eternals watch over humanity, protecting them from the Deviants - the Eternals' opposite numbers who sometimes experience "excess deviation" and become monstrous threats which the Eternals are driven to kill by their internal programming.
In Eternals #12 by Kieron Gillen and Esad Ribic, it's confirmed that Thanos was created through a combination of Eternal and Deviant DNA, as part of his mother and father's grand plan to expand the Eternal race beyond the initial hundred beings created by the Celestials. However, the series' focus on how the Eternals are forced to address excess deviation opens up a tragic possibility given Thanos' true nature. Eternals are forced to immediately kill those Deviants who mutate into excessive threats, having no choice in the matter, and by most sensible measures, Thanos is a mutated Deviant threat. It's possible he's obsessed with Death because his Eternal programming is prompting him to try and destroy himself.
This isn't spelled out explicitly in the issue, but it would be a perfect marriage of Thanos' past and present. Jason Aaron and Simone Bianchi's Thanos Rising revealed that Thanos was tempted by Death from childhood, sensing something was deeply wrong with him, and even vivisecting his mother to find its cause. Eternals #12 confirms that Thanos was given Deviant DNA so he could overcome his Eternal programming, allowing him to step beyond the Celestial's parameters, which govern the rest of the species. However, it's been suggested that wiping out excess deviation is the Eternals' most deeply rooted programming, and it's possible that while Thanos isn't forced to obey like other Eternals, he still possesses the instinct that any dangerous Deviant must be destroyed, even when he's the danger in question.
Thanos' obsession with Death includes his own desire to die. This has happened before, but Thanos has become so important to Marvel's cosmic ecosystem, the natural forces of reality keep finding ways to bring him back. Indeed, part of the death and destruction Thanos has rained down on the universe has been an attempt to wipe out all other life so he himself can die.
In the past, Thanos has been described as allergic to life, and residual Eternal programming conflicting with his Deviant nature would make this more or less literal. If Thanos does indeed feel a warped version of the imperative to erase excessive Deviant mutation, it would explain why he craves death, as it's the only way to silence the instinct that his own existence is a terrible wrong that needs to be addressed. Hopefully, Marvel will make this idea explicit in future, as the dark twist would explain Thanos' past and motivations in an incredibly compelling and tragic way.
Eternals #12 is available now from Marvel Comics.
