Diana Prince (Gal Gadot) was against the resurrection of Superman (Henry Cavill) in Justice League and her experience in Wonder Woman 1984 explains why. The Themysciran Princess' solo adventures continue with the upcoming Patty Jenkins film that will take place during the height of the Cold War. This time, Diana faces a new set of villains in Barbara Ann Minerva/Cheetah (Kristen Wiig) and Maxwell Lord (Pedro Pascal). Since the movie takes place in the '80s, its narrative could help fans better understand why Diana was initially very adamant about not wanting to bring back Clark Kent in Justice League despite it being for a good cause.
Zack Snyder's Batman V Superman: Dawn of Justice ended with the death of Superman after he sacrificed his own life to defeat Doomsday. This functioned as the springboard for Justice League, which featured the arrival of a more dangerous threat in Steppenwolf. With no Man of Steel to lead the pack, Bruce Wayne/Batman (Ben Affleck) and Wonder Woman assembled a team of superheroes that would help them take on the villain. However, they eventually realized that all of them together may not be enough. This is where the idea of bringing back Superman using the Mother Box came up.
Ultimately, everyone in the team agreed with the suggestion with Bruce making a great case about why they need to do it. Batman argued that aside from Superman being a great help in fending off Steppenwolf, he also believed that he could restore hope to mankind. However, Diana, being the only hero who knew first-hand what went down between him and Clark in Batman V Superman also saw his personal motive — he was still racked with guilt from Clark's death that resurrecting him could potentially ease that. Looking at what Diana's experience will be in Wonder Woman 1984, this may not be the only reason why she's against the idea.
The upcoming Jenkins sequel will also bring back Steve Trevor (Chris Pine) — Diana's love interest from the original Wonder Woman film who, like Superman, died sacrificing himself for the greater good. His return is one of the central mysteries of Wonder Woman 1984 as the character is front and center in the movie's marketing. Trailers have shown him in an emotional reunion with Diana who is still in love with him decades after his presumed death. Given what happened in World War I, it's clear that Steve's return isn't from natural causes.
Theories of him surviving the crash or escaping the aircraft before it exploded don't line-up with his physical state in the '80s — barely aged and seemingly have no idea of how the world has evolved since the early 20th century. With the character nowhere to be found in the modern DCEU world, there's a good chance that the end of Wonder Woman 1984 will once again see them part ways. To make it worse, their separation this time could be more difficult and ugly since it could be borne out of Diana realizing that messing with the natural order of things always has consequences. So while she doesn't want to let Steve go, she needs to accept the fact that he's dead to resolve any harmful side effects brought about by his forced return.
Given this potential experience in Wonder Woman 1984, Diana has learned that playing god and messing with the natural order of things has its unintended consequences. "Technology like any other power, without reason, without heart, it destroys us,” she argued, referring to using the Mother Box to resurrect Clark in Justice League could lead to a catastrophe. What she went through in the '80s also makes her more sensitive to Bruce's jab about Steve which was initially perceived to just be about his death in the original Wonder Woman film. Unfortunately, as she was outnumbered by the rest of the team, she had no choice but to get on board with it. In any case, at least Bruce somehow listened to her argument, enough for him to come up with a contingency plan in case Superman's resurrection went awry.