The original story made it very clear why Hatake Sakumo committed suicide. Back then, he abandoned the mission for the sake of his comrades, yet the comrade he saved stepped forward and condemned him, believing that he should not have saved him at the cost of abandoning the mission. The mission's failure had caused the Land of Fire tremendous losses.
That was why Kakashi had once stubbornly believed that comrades could be abandoned for the sake of completing a mission.
It was only after Obito Uchiha learned the truth that he told Kakashi that comrades came before missions.
It was precisely because Hiruzen had always demanded that the true reason behind Hatake Sakumo's suicide be concealed that Sakumo had remained a heroic figure in Konoha.
Konoha's villagers, as well as the vast majority of its ninja, had never known the truth. They had always considered him a hero, a legend.
If the villagers had truly learned the truth, Kakashi would not have merely become gloomy. He might even have been driven by rumors to want to commit suicide, just like his father.
It was precisely because Hiruzen had always insisted that this secret could not be made public that Kakashi had been able to grow up in peace.
The reason he had been so sullen before was because his father had committed suicide after failing a mission.
Kakashi had always looked up to his father as a role model and always believed he was incredible.
Yet he had actually committed suicide over the accusations of a few comrades. Kakashi had never been able to let go of that.
That was why he had always been gloomy.
He only changed after Obito Uchiha helped him come to terms with it.
And in truth, Kakashi had not been gloomy at that time.