When Thor became worthy of Mjolnir, he also proved his one true love. The greatest son of Odin, Thor has lived a long life - and that naturally means he has had many loves. Even Thor: Love & Thunder alluded to this, with a scene in which the Odinson was shown making love to a wolf woman on the back of a woman wolf. Amusingly, the wolf woman in question was played by Chris Hemsworth's wife, Elsa Lafuente Medianu.
Fans tend to assume Thor's greatest love is in fact Jane Foster. In the comics, Jane is a human nurse who worked alongside Thor's alter-ego, Dr. Donald Blake, and she fell hard for the God of Thunder. The MCU's version is a little different, an astrophysicist whose work on Einstein-Rosen bridges earned her a Nobel Prize. Both versions of Jane Foster ultimately proved themselves worthy to wield Thor's enchanted hammer Mjolnir, which seems to indicate these mortals stand equal to Thor in some way. And yet, for all that's the case, anyone familiar with the Norse legends will know Thor's one true love is in fact the Lady Sif.
This truth is demonstrated in the Thor: Son of Asgard series, which reveals how Thor first became worthy of Mjolnir. By C.B. Cebulski (under the pseudonym 'Akira Yoshida') and by Greg Tocchini, the series explores Thor's early days in Asgard and reveals how he finally became worthy of Mjolnir. The God of Thunder had long been eager to claim Mjolnir as his weapon, convinced that by doing so he would prove himself Odin's true heir, and yet he had been unable to lift it. All that changed when Sif was kidnapped, and Odin told his son to remain behind at Asgard. Frustrated, Thor sought out Mjolnir once again, and this time it allowed him to lift it. The enchanted hammer clearly made that choice because, this time, Thor was not trying to use it out of selfish desire or ego; he was motivated purely by love.
This fits well with Marvel's concept of worthiness. A being who is truly worthy must be willing to set aside their own self-interest and choose to fight on behalf of others. Proud and arrogant gods tend to believe they do not need to love, choosing to close off their hearts to others. But in allowing himself to love the Lady Sif, Thor made himself more vulnerable than ever before. To love is to risk, and only a god who is willing to risk their own heart can ever be worthy.
Thor's origin story means there is a bond between the God of Thunder and the Lady Sif that can never be repeated. Thor's love for Sif was what made him worthy to wield Mjolnir in the first place, meaning his ion for her lies at the very core of his story. No matter which other maiden catches Thor's eye, Sif will always be unique.
