A Few Theories
Theory 1: SeokJin's Memories
Some fans think that SeokJin actually loses his memories of past timelines when he travels through time. One of SeokJin's main symbolic possessions is his camera, which can be seen many times throughout the HYYH Prologue, the Euphoria Music Video, the Run Music Video, and others. He takes these pictures to remember what happened in an alternate timeline (in the SAVE ME webtoon, SeokJin maps everything out and writes down information on sticky notes to determine who would be where at what time and how and when he or whoever should save them, that way he could find the best possible way to save everyone). That also may be why he looks confused at times (e.g. zooming into his face in the Run music video) because he doesn't remember/recognize something from a current timeline when it's out of his memory.
Theory 2: Witness JiMin
Throughout the story, there are two characters who are labeled as the 'sick' or 'hospital' friends, HoSeok and JiMin. JiMin has frequent seizures and has been to 6 different schools because of this. Whenever he lands in the hospital again, he stays for a few months, and then switches schools occasionally. The Grass Flower Arboretum has some significance in JiMin's life, it is seen in the SAVE ME webtoon, Jimin's short film titled Lie, and in The Notes 1. He may have witnessed the murder of a child at this arboretum, which makes sense why it brings bad memories. In The Notes 1, there is a prologue describing SeokJin in his elementary school days. His friend's father owned a company that unfortunately shut down. During class, a few men came in looking for the friend, so SeokJin grabbed his hand and ran outside, away from the school, back to SeokJin's house. SeokJin's father was a strict man, and when the men came knocking on the door, SeokJin's father did not let SeokJin protect his friend. The men took him away. The next day in class, his friend was not there, and the teacher informed the class that he had switched schools.
JiMin once attended a school picnic on a cloudy day near the Grass Flower Arboretum. When they were ready to leave, his teacher told him that she'd be right back and told him to wait. JiMin didn't wait and thought he knew his way and left the arboretum. After a while, it began to rain, and JiMin lost his way. The only thing he saw around him was a sign reading Grass Flower Arboretum. Many people theorized that here he witnessed the murder of SeokJin's friend (in the SAVE Me webtoon, there is a red puddle next to the sign). Whenever he sees or thinks of the sign, bad memories flood his mind, and he occasionally starts seizing.
Theory 3: The Girls in the Love Yourself Highlight Reel
In the Love Yourself Highlight Reel, each of the boys is seen with a different girl (with the exception of HoSeok and JiMin, who are seen with the same girl). Fans speculated that the girls could all represent the boys themselves. In the Bangtan Universe, there were three very distinct friendships: NamJoon & TaeHyung, YoonGi & JungKook, HoSeok & JiMin. The girl who was with NamJoon represented TaeHyung, and the girl who was with TaeHyung represented NamJoon. Same with the others.
However, HoSeok & JiMin were seen with the same girl. The girl in this theory represented JiMin. HoSeok's quote from one of the Highlight Reel posters was "I am happy as long as you can shine" indicating that he was a bit more selfless and that JiMin was more self-centered (because he was always focusing on himself and how he was never good enough). In the Highlight Reel, JiMin is dancing with the girl when they both fall. HoSeok tries to save the girl (representing JiMin) and runs with her in the rain, which is why JiMin is wet even with an umbrella.
In the RUN music video, YoonGi and JungKook get into a fight, and at the end JungKook gets hit by a car. He is seen recovering from the accident in the hospital in the Highlight Reel. JungKook needed a donor (I'm not sure for what), and YoonGi gets a call. YoonGi is determined to save JungKook even though he will die if he is the donor, which is why he can be seen struggling with his girl (representing JungKook) who is begging him not to give up his life for JungKook. YoonGi goes anyway, which is why the girl who was at the hospital with JungKook (representing YoonGi) is no longer there when he was able to stand up.
In the I NEED U music video, TaeHyung murdered his father (also seen in the webtoon). On an alternate timeline that SeokJin travelled to, NamJoon murdered TaeHyung's father. TaeHyung is seen surrendering to the police after spraying graffiti on some municipal buildings, leaving the girl (NamJoon's representation) behind. Even though these two events aren't really linked, fans think that TaeHyung went to jail for NamJoon, and when he got out NamJoon couldn't face him anymore (NamJoon's quote from the Love Yourself Highlight Reel posters, "Always looking at your back") because he didn't have the courage to. NamJoon wasn't brave enough to talk to the girl (TaeHyung's representation) and instead simply leaves a hairtie on her backpack while she is asleep. TaeHyung didn't ever see the girl (representing NamJoon) again because NamJoon himself couldn't bring himself to.
SeokJin is in the middle of all of this, and his girl actually represents all of the boys. Since every time he couldn't save one of them, he would go back to April 11th, he saw many deaths and many disasters during different timelines. His whole world fell down when number one, he realized that his friends were either in jail, in the hospital, or dead (I NEED U) and number two, he saw the girl get hit by the delivery truck (Love Yourself Highlight Reel). This is also why even when none of the boys were failed, SeokJin still went back in time, because the girl representing ALL of the boys was failed.
References
The Most Beautiful Moment in Life: Prologue, YouTube comment section
Love Yourself Highlight Reel, YouTube comment section
'Euphoria: Theme of LOVE YOURSELF Wonder'