It turns out that transforming into a Super Saiyan has nothing to do with training or anger, but rather a biological factor. In a recent interview published in the manga magazine Saikyō Jump, Dragon Ball creator Akira Toriyama explains the secret of the Super Saiyans.
The Biological Secret of the Super Saiyans
If you are following Dragon Ball Super, whether the manga or the anime, you have probably been surprised by the fighting level of the Universe 6 Saiyan warriors, who can easily transform into the Super Saiyan state. To resolve this doubt, in a recent interview in Saikyō Jump, Akira Toriyama revealed the secret of the Super Saiyans. To become a Super Saiyan, the Saiyan's body must contain S-Cells. Once a certain amount of S-Cells is reached, anger can trigger a massive increase in these cells, causing a physical change known as the Super Saiyan transformation.
“Having a good spirit is the best way to increase S-Cells,” says Toriyama.
“However, it is not possible to reach the necessary amount of S-Cells to transform into a Super Saiyan just by having a kind spirit, so a certain amount of battle power is really necessary.”
That would explain why Goku could transform into a Super Saiyan so easily. Most Saiyans have some S-Cells, though not a large amount. Therefore, one of the reasons Goku's and Vegeta's children transformed into Super Saiyans is that they inherited these cells. They have the genetic structure to transform, in addition to having been raised on Earth, a more peaceful and gentle environment than Planet Vegeta; the same goes for the Universe 6 Saiyans, who come from a gentle and pacifistic people.
https://www.youtube.com/embed/pMoCCaLnjggYamoshi, the First Super Saiyan
Akira Toriyama also revealed that the first Super Saiyan is a Saiyan named Yamoshi, who was kind-hearted. He and his five comrades began a rebellion, but they were outnumbered. With this, Yamoshi was forced to transform and became the first Super Saiyan. Unfortunately, Yamoshi ran out of energy and was defeated, giving rise to the legend of the Super Saiyan. Later, Yamoshi's spirit wandered in search of six pure-hearted Saiyans. This is the reason a Saiyan must first undergo a ceremony before becoming a Super Saiyan God, as seen in the film Battle of Gods.
Source: Kotaku
