Why did Thor bring Mjölnir to the main timeline?

Question

In Avengers: Endgame, why did Thor decide to bring Mjölnir from his original timeline to the main one?

The part where he stretches his arm to summon the hammer was just his will to know whether he's still worthy or not, which he was. So why did he bring it back with him to the present time?

Thanos was long dead and there was no war to be fought at that point of time. There was no way he could know of the impending danger i.e. Thanos traveling to future.

What were his plans after the snap? To keep both the weapons in his inventory, while the Thor in the alternate timeline pulls his hair out while searching for his dear Mjölnir?

Answer 1

The most obvious answer is nostalgia... But I think we can take the analysis a bit further than that.

It was just his will to know whether he's still worthy or not, which he was.

Thor has a "special and intimate relationship with this hammer". He's wielded it in every battle, and at the end of the first Thor movie, Mjölnir coming back to him meant he was worthy again. The last time he saw Mjölnir, it got destroyed from his hidden Hellish sister, and it took him a long time to understand he wasn't the "God of hammers", and could operate without it. Still, you can't beat the nostalgia.

By Endgame, Thor is in a bad place. As Paul developed in this excellent answer, Thor lost pretty much everything by Ragnarok: his mother, his father, his hammer, his freedom, his cape, his hair, his eye. By Infinity War, it's worse: his brother died, as did half of his people and the rest of the world, that as a superhero he had a moral duty to protect. By Endgame, you'd think it couldn't be worse, but it can: Thanos destroyed the Stones, nobody can do anything about what happened. There's just no point anymore, and Thor must be pretty depressed.

Make all the "fat jokes" you want (or better yet: don't), Thor knows he's not "fine". So when the gang is back together again, with a purpose, hope, and he is given the chance to tell his mother goodbye (sort of) and he discovers he's still worthy, when he had doubts he could (despite the after-fact "I knew it!")? Immediately followed by troops charging with Norse artillery to thrash his raccoon pal and him? Thor won't stop to think about it. He's whole again, and Mjölnir is part of that.

Thanos was long dead and there was no war to be fought at that point of time.

Irrelevant; Thor is now "back in action", and action means the hammer, even if the plan is only to unsnap and hope everything works out fine.

Plus, heroes always prepare for war, right? That's what they do, especially Thor, who is almost born for fight.

What were his plans after the snap? To keep both the weapons in his inventory, while the Thor in the alternate timeline pulls his hair out while searching for his dear Mjölnir?

Thor and plans, not two words that go along well. MCU Thor, even in his best day, was never that much of a thinker/planner, and the time travel consequences are probably not reaching him. For all he knows, as they have the Stones, if anyone says something he'll be able to return Mjölnir in no time (pun intended).

Answer 2

I think Thor was following along enough to understand the plan of "borrowing" the Stones and returning them, such that they would only be gone for seconds.

And it dawned on him that he could do the same thing with Mjölnir.

Given his adventurous, "why not" attitude, he just does it.

And yes, indeed, the Avengers also "get" that the hammer can be returned when the Aether is returned, and are simpatico with it. Note that Stark and Banner don't yell at him for doing it, and they certainly would if it was a problem.

Answer 3

Thor is a warrior.

There are always enemies to fight, not only Thanos.

So when realizing there's still purpose to life, he simply wanted his favorite weapon to go with him, likely without thinking much about past or future.

As for his plans, he says so himself in the end of Endgame:

Valkyrie: What will you do?

Thor: I’m not sure. For the first time in a thousand years, I have no path. I do have a ride though.

Answer 4

One possibility is that he was taking to test a hunch that Captain America was now worthy (for one or more reasons), as evidenced by the celebratory "I knew it!", when that happens. Contrast that with the anxiety/doubt he displayed when Cap attempted and failed to pick it up in Avengers 2.