WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 168, "Training Begins!" now streaming on Crunchyroll.
In Naruto series, Kakashi quickly became one of the most popular shinobi because of his skill in battle, his sense of humor and how awesome he was teaching the likes of Naruto, Sakura and Sasuke. The "copy ninja," thanks to his Sharingan, also lent some to mystery to his background, which definitely earned points with fans.
However, bit by bit, more of Kakashi's history came out and it proved to be quite dark, which justified it being locked away. Come Boruto years later, when he's older, wiser and more content in retirement, Kakashi now reveals his biggest regret in Konoha.
Kakashi lived a very tough life, losing his mom at a young age, with his dad, Sakumo, quickly following suit. But it's Sakumo's story that really stabs at Kakashi's heart, because he idolized his dad and wanted to be like him. However, Sakumo abandoned a mission to save some teammates, only for it to backfire when he got back to Konoha. Everyone treated him like a disgrace, including the people he saved, because he broke ninja code.
Sakumo became depressed and killed himself, leading to Kakashi closing himself off. He'd end up working with the ANBU military unit, killing from a young age and becoming mentally scarred. Even though he trained under Boruto's easygoing grandfather, Minato, before this, Kakashi remained frigid, culminating in him "killing" his teammate and the girl who loved him, Rin. She impaled herself on his Chidori, a lightning blade made with his fist, as she was turned into a weapon of mass destruction by a rival village and sent back to Konoha as a bomb. It's her loss that broke Kakashi forever.
And as Kakashi tells Boruto now, he regrets growing up in that era where child soldiers were the norm. As Boruto trains under him following a recent loss in battle, he can tell the kid's a brat, annoying and spoiled, but he's hard-working like Naruto, so he wants to pass wisdom on for Boruto to appreciate what he has, even if it's as small as his sister packing lunch. Boruto's heart is in the right place so Kakashi educates him on the past and present Konoha, while harping on the concept of family. He considers Boruto lucky because now, he sees so many families like Boruto's happy in a land where there's love, unity and peace. He's envious, especially after all the Great Nations have finally come together, something he played a part of as he was Hokage before Naruto. Still, Kakashi wishes he grew up as a kid in this current era where kids aren't turned into weapons, considering Boruto lucky to have so many people who cherish and love him.
Konoha no longer has to militarize its students the way he did with Naruto's team, or the way others before him were used, per Obito and the Uchihas, too. There were many deaths and sacrifices to create this era but Kakashi wishes he didn't have to be part of those wars -- he simply wishes he could enjoy the spoils of victory. And as he speaks to Boruto, you can sense the sadness and trauma he's been masking, throwing fans back to why he became so jovial after Rin's death. He was in denial, burying away that morose part of him, and as he got older, he tried to become the child he never got to be. It explains why he played pranks on his students and, as he tells Boruto, it's sad that you can't choose the era you were born in. Hearing all this, Kakashi loyalists definitely would have loved to see him as a kid growing up in a Konoha where science, tech, education and diplomacy have created a veritable ninja utopia, although that seems to be changing with Deepa and Kara coming to light.