One of Marvel's most unlikely feuds ever is Thanos' hatred for the Merc with a Mouth, Deadpool. However, the Mad Titan believes he has a valid reason for despising the talkative antihero, as Wade Wilson stole his greatest love - leading Thanos to curse the fourth-wall-breaking jokester with immortality to split him up from Death. Thanos was truly driven mad by Deadpool.
Deadpool had what Thanos always desperately craved, the attention and affection of Death. When he was experimented on as part of the Weapon X Program in Deadpool/Death '98, he was near-death on many occasions, where he met Death and developed feelings for the cosmic being. Finally, Deadpool was able to see her, despite not being officially declared dead. Surprisingly, Death fell hard for Wade Wilson as well, as the pair shared a surprising romance. One person who didn't enjoy the odd couple was Thanos - granting Deadpool immortality so he couldn't see her. However, his plan didn't work.
In Deadpool vs. Thanos by Tim Seeley, Elmo Bondoc, Ruth Redmond, and Joe Sabino, Marvel's Infinity Gauntlet-wielding villain teams up with the unkillable hero to track down Death. It's revealed Death is being kept behind lock and key by the living embodiment of Eternity, and when the pair track her down, she immediately turns her attention to Deadpool. Thanos can only watch in horror as she calls Wade Wilson her "love," leading the Mad Titan to attack him.
Thanos might hate Deadpool, but his unhealthy obsession with Death has come back to bite him on multiple occasions. She has rarely expressed even the slightest interest in the Mad Titan, even refusing to talk to him on numerous occasions. Even when he snapped half of existence from reality during the Infinity Gauntlet saga, Mistress Death just stood by and seemed largely unimpressed by his actions. She turned on him towards the end of the story, ensuring his defeat. Simply put, she's just not into Thanos - no matter how many times he conquers the universe.
Part of what makes Deadpool and Death's unlikely romance so great is Thanos' jealousy. Wade Wilson is pretty much the villain's opposite, as his carefree attitude and pure silliness are nothing like Thanos' ultra-serious quest for power and Death's love. Deadpool constantly surprises Death because he isn't always trying to impress her. Instead, he captures her attention simply by existing. Meanwhile, Thanos' tough-guy attitude seems to do nothing for her. Thanos hates Deadpool not because he's some great hero like Thor or Hulk with whom he's had epic battles, but because he took the one thing he'll never be able to attain: Death's attention and love. Stay humble, Thanos.