The following contains spoilers for Jujutsu Kaisen Chapter 65, "Hidden Inventory" onwards, by Gege Akutami, John Werry and Snir Aharon, available now in English through Viz Media.
Gojo Satoru has not always been as strong as he is now in Jujutsu Kaisen. When he was a student at Jujutsu High, he only knew how to use one aspect of his technique and was unable to unlock his true potential until midway through his second year. Despite this, Gojo and his classmate Geto Suguru proclaimed themselves "the strongest" whenever they worked together.
The duo were an unstoppable force but when Gojo's full power was realized, Geto began to spiral and the two drifted apart. While the pair's friendship wavering was ultimately traumatic for both, their bond falling apart showed how Gojo isn't as untouchable as Jujutsu Kaisen fans may think.
Geto and Gojo's Mission to Escort Riko Amani to Jujutsu High
When Geto and Gojo were in their second year, they were tasked with a very important mission: to safely transport the Star Plasma Vessel, Riko Amani, to Jujutsu High to fulfill her duty and merge with Master Tengen. Unbeknownst to her, Geto and Gojo decided that if Riko wished to live out her life instead of merging, they would allow this and face the consequences themselves -- even if it put their lives at risk.
The pair battle countless curse users on their mission, exhausting themselves with the constant use of their techniques, but they get Riko safely to Jujutsu High. However, once in the school's barrier, the worn-down pair are confronted by Toji Fushiguro, an assassin assigned by the Time Vessel Association to stop the merger. Toji and Gojo battle as Geto and Riko flee, and Gojo is defeated and left on the brink of death. Toji catches up with the others, and just as Riko decides to live out her life with the ones she loves, Toji shoots her.
Geto and Gojo Never Recovered From the Riko Incident
This event in Jujutsu Kaisen caused trauma for both Geto and Gojo; the strongest sorcerers were defeated by a human with no cursed energy. They each blame themselves for Riko's death and slowly grow apart. Gojo in particular is wracked with guilt, as he was the one tasked with fighting off Toji while the others got to safety. His battle with the assassin and glimpse of death unlocked Gojo's full potential, making him the unstoppable force recognized in the anime.
Gojo became the strongest on his own and so was sent on solo missions, while Geto did the same. The pair didn't talk for a long time after the Riko incident and were unaware of each other's feelings. They tried to deal with the trauma; Gojo immersed himself in training to suppress the memories while Geto tried to work hard -- but what happened slowly ate away at everything he once believed in. Gojo was becoming stronger, filling himself with confidence that nothing like the Riko incident would ever happen again. Geto was suddenly overshadowed, coping with loneliness and inferiority.
Losing Geto Showed Jujutsu Kaisen Fans That Gojo Is Still Human
In the end, Gojo Satoru was satisfied by his progression and immense achievements while Geto was satisfied by nothing. The latter lost his way and spiraled into a negative state, hanging on to hope by a thin thread. Gojo found himself losing his best friend to the side of evil, with Geto ending his life with his own hand in Jujutsu Kaisen 0.
It's easy to question why Gojo didn't do anything when Geto was clearly struggling. Even in some of their final moments in high school, he joked about his weight loss but doesn't pry as a worried friend might. However, this is something that happens in real life. People get distracted by their own problems, emotions and worries; despite how much love may be held for another person, sometimes they get put on the back burner. This is exactly what happened with the strongest duo. Gojo disappeared when his friend needed him most, and even though his bond with Geto was strong, the two still drifted apart. Unfortunately this is normal -- but the consequences are often tragic.
This forces the audience to knock Gojo off his unattainable God-like pedestal, because he made a crucial yet very human mistake. He realized his blindness toward Geto too late and it ended in disaster, which makes Gojo seen as less of a God and more of a human. Had the pair worked through Riko's death together, they could have stayed the strongest together. Instead, Gojo faces the burden of being Jujutsu Kaisen's strongest alone.