Tony Stark a.k.a. Iron Man (Robert Downey Jr.) stopped Thor (Chris Hemsworth) from doing the first snap after the time heist in Avengers: Endgame — because of Thor's mindset at the time. Five years after their loss to Thanos (Josh Brolin) and the destruction of the Infinity Stones, the remaining MCU heroes banded back together when the idea of the time heist came up thanks to Scott Lang/Ant-Man (Paul Rudd). The complicated mission demanded they travel back to different points in the MCU history to collect the Infinity Stones before the Mad Titan got them. Despite a couple of snafus along the way, they were ultimately successful in securing the crystals, giving them the opportunity to bring back those who were dusted by the villain's decimation in Avengers: Infinity War.

Using Stark's nano-tech gauntlet, Smart Hulk (Mark Ruffalo) wielded the power of all six Infinity Stones and brought back the victims of Thanos' snap. Prior to the collective decision to let the green creature don the metal glove, there was a brief commotion among the heroes. Thor insisted that he wanted to do the snap and begged his colleagues to allow him to do it so he could contribute to the cause. However, they collectively decided that the God of Thunder wasn't right for the job.

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Being a demi-god, Thor is capable of using all six Infinity Stones at the same time, perhaps even more so than the Hulk. Granted that he did gain weight in the five years since he ruthlessly beheaded Thanos in the garden after learning that he destroyed all the gems, he's still physically strong. This is evinced by the way he partook in Endgame's climactic battle. Tony was particularly adamant in not letting Thor do the snap, but instead of simply thinking about his fellow Avenger's well-being, there's a deeper reason why he didn't want the God of Thunder to do it.

Fat Thor sits in New Asgard drinking beer in Avengers Endgame (1)

Tony initially refused to be involved in the time heist — not because he was skeptical that it could work, but because he didn't want to risk losing the life he'd built over the last five years. This makes sense considering that this was the only time he actually had a semblance of a normal family dynamic, considering his tumultuous relationship with his father, Howard Stark (John Slattery) growing up. Tony knew that to be able to secure his future with his wife, Pepper Potts (Gwyneth Paltrow) and daughter, Morgan (Lexi Rabe), there should be a clear directive to the Infinity Stones with what the wielder wants to happen. As it is, snapping or any other physical act doesn't activate the power of the Infinity Stones — it's the mind that mandates what the stones do. So, while Thor is physically capable of wielding the power of all elemental crystals, he wasn't in the best of mindset to send them clear instructions with what needed to happen.

Throughout the film, he was unstable, going through emotional breakdowns every now and then as he continued to grapple with his catastrophic failure to stop Thanos in Infinity War. Knowing a thing or two about mental health issues, having suffered PTSD after the Battle of New York, Tony didn't want Thor to potentially mess up what could be their only chance in using the Infinity Stones. This is also the reason why Tony gave Smart Hulk a final reminder before the latter put on the gauntlet saying: "you remember, everyone Thanos snapped five years ago, we're just bringing them back now, today. Don't change anything in the last five years."

Ironically, prior to Avengers: Endgame, Thor would've been the heroes' unanimous choice to do the secondary snap as he was both physically and mentally-equipped to accomplish the job. Without the five-year-break that allowed Bruce Banner to meld his two different personas into Smart Hulk, they would've been stuck with a mindless green rage monster. Given this, there's no way any other Avenger would want him putting on the Stark gauntlet, especially given the lengths they went through to acquire the Infinity Stones.

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