When facing Thor, Fury whispered something into the God of Thunder's ear that not only shocked him but left him unworthy to lift his hammer. This event would serve as the catalyst for Jane Foster to claim the hammer, becoming the Mighty Thor, and for plenty of villains to rise in the aftermath.
Even more people don't know the hammer has a name. During Marvel Comics Original Sin story-line we saw Thor Odinson become unworthy to wield the hammer. During a fight, Nick Fury was able to utter a simple sentence in Thor's ear. Whatever was said to Thor caused him to be unworthy to wield Mjolnir.
In the comics, the original Thor (let's call him “Thor Classic”) finds himself unworthy of wielding the Mjölnir hammer, which means it has fallen from his grasp and cannot be lifted. There it stays, stuck on the moon, until a woman appears and picks it up—finding herself imbued with all the powers of a thunder goddess.
During the events in the Original Sin storyline in 2014, Nick Fury, with the powers of the Watcher, whispered something to Thor that caused him to lose the ability to use the Mjolnir, making way for another worthy character that soon took the name (and powers) of Thor: Jane Foster.
SPOILER WARNING for The Unworthy Thor #5
In The Unworthy Thor #5, which released in comic shops today, the words Nick Fury said were finally revealed. "Gorr was right." Three simple words that undid the God of Thunder.
Starting A War With Jotunheim
Because he was not big enough to let the issue go, he reignited war with the Frost Giants. Even after this was accomplished, Thor failed to see the stupidity and selfishness of his actions. He mocked his father for seeking peace rather than courting war as he does.
Following his defeat, Thor began to refer to himself as merely Odinson, wallowing in his loss and humiliation. It wasn't until 2016's Unworthy Thor #5, by Jason Aaron, Olivier Coipel, Kim Jacinto and Pascal Alixe, that Thor finally revealed that the words Fury had said to him were simply, "Gorr was right."
Other Marvel characters have lifted Mjolnir not through worthiness but through technicalities such as absorbing Thor's powers, including the Air-Walker, Awesome Android, Magneto, Rogue, Wonder Man, and Doctor Doom.
Hence, Mjolnir determines worthiness based on Odin's definition of worthiness. However, since Mjolnir also has a conscious being within it, in the form of God Tempest, another likely explanation is that it is God Tempest that determines the worthiness.
In a moment of wistful affection for his girlfriend, Thor asked his trusty hammer to always protect Jane, and in doing so unwittingly enchanted Mjolnir to do just that. Just like Thor's father Odin once enchanted the hammer to only be lifted by someone who is worthy, Thor cast a spell that let Jane use its powers.
Gorr planted seeds of doubt in Thor's mind, forcing him to question whether gods truly are as noble and benevolent as they seem. Nick Fury seizes on that doubt in 2014's Original Sin, telling Thor "Gorr was right" and rendering the god of thunder so disillusioned that he's no longer able to lift Mjolnir.
Ultimately, Love and Thunder confirms that the God of Thunder is still worthy as Thor wields Mjolnir once more, but the delay both helps Jane and proves Thor's worthiness on a much deeper level.
Why Thor Has His Eye Back In Love and Thunder. Several shots in Thor: Love and Thunder give closeups of Thor's eyes and show them to both be a piercing blue with no discernable difference. It is possible that this is a simple filmmaking mistake, or was a change made to avoid the hassle of keeping up the effects.
Thor loses his eye during the earlier parts of the final battle sequence with this goddess. She casually and effortless beats him around the throne room, and then the moment happens. Hela strikes Thor across the face with her sword, literally slicing the eye from out his head right then and there. Brutal.
Fat Thor's depression led to his mental and physical decline, which is why he was less powerful in Avengers Endgame than in Ragnarok and Infinity War.
Why is movie Thor weak in strength compared to the comic Thor? Because when Thor was introduced in the MCU The Avengers were already planned, and The Avengers tried to avoid the mistake that DC would later make: Having one character (or two if you count Hulk) that makes the rest of the team look unnecesary.
If fans wait to see Peter Parker lift and use it in MCU, they may be disappointed. Despite his strong moral sensibility and strength of character, Peter lacks the ruthlessness that destroys all evil. For this slight defect in character, Mjolnir will never find this Spider-Man worthy of it.
Why was Groot able to lift the new Mjolnir in Avengers: Infinity War? The “new Mjolnir” is an entirely different weapon called Stormbreaker. It has no such enchantment attached to it that states only the worthy can wield it, therefore Groot can pick it up.
Avengers: Age Of Ultron Confirmed Iron Man Wasn't Worthy
Tony Stark was the second one to try, even joking about ruling Asgard if he lifted the hammer and what he would do as king, but his always overly-confident attitude quickly changed when he realized he couldn't lift the hammer.
Doctor Doom
Not everyone who has lifted Thor's hammer was worthy to hold Mjolnir. A perfect example of this is Doctor Doom, one of the world's most dangerous villains. While Doctor Doom is a master of magic and science, it was his luck in finding the hammer that allowed him to wield it.
Forged by dwarves in the heart of a dying star, the weapon is enchanted so that it can only be lifted and wielded by those who are worthy of its powers. Both in the comics and the MCU films, many characters other than Thor himself, both heroes and villains, have been able to lift Mjolnir.
Odin's “worthiness” enchantment to Mjolnir was made only during the events of the first Thor movie. Thus, Hela being able to handle/wield the Mjolnir before had little to do about her being “worthy.” You could say part of the reason why Odin added the enchantment is to prevent another Hela from using the hammer.
Thor's main fear is losing his worthiness because it would mean he isn't fit to protect Earth or lift Mjolnir.
In their attempts, Jane tells him, "She won't be alone," signaling the fact that Thor would take the girl under his wing. Finally, before she passes, she tells Thor to keep his heart open, bringing Jane and Thor's story to a heartbreaking close.
Originally there was no restriction placed on the hammer for who can use it but after Thor's reckless behavior, Odin Borson placed an enchantment on it. This spell read "Whosoever holds this hammer, if he be worthy, shall possess the power of Thor." Only those deemed worthy to lift Mjolnir can.