The Z saga began with the arrival of a strange warrior on Earth, named Radditz, who revealed the true origin of Son Goku, or better, Kakarrot.
Until that moment, Master Mutenroshi’s disciple saw himself as a human which the only abnormality was his super strength and a monkey tail, however, with this stranger’s arrival, Goku finds out that he is one warrior of an alien race, the most powerful of the universe: the saiyans!
Radditz reveals to be Goku’s older brother, whose name is Kakarrot, and whose home planet had been destroyed by a meteor shower, and at the present moment, there were few remaining saiyans: with Goku, only four were still alive! The saiyans job was to exterminate the inhabitants of different planet in the galaxy and then sell them at a high price for other alien race that were interested on it.
As everyone might know, Radditz wanted Goku to help him and the other two saiyans to “clean” a planet that was presenting heavy resistence. Goku denies, Radditz hits him. Piccolo and Goku face Radditz and defeat the invader, and then he alert that the two other saiyans are far more powerful than him. OK.
Months later, we see ourselves at Planet Namek, already in the great emperor of the universe, Frieza. The Eldest helps Krillin and Gohan, giving them his Dragon Ball and leveraging their kis. The patriarch of namekuseijins learns about the death of Piccolo, son of Katatsu, and makes question that later would be answered: “Killed by a saiyan…? The saiyans can be formidable, but not enough to defeat that kid… Was this saiyan a super saiyan…?!”. The Eldest also said that if Piccolo had not been separated into two, to get rid of the evil part (that became Piccolo Daimao), he wouldn’t have been defeated by any ordinary saiyan.
What would be this super saiyan? The mystery starts to be revealed with Vegeta talking about this when Goku arrives at Namek and face Ginyu Special Forces easily.
From that we find out about the legend of the most powerful warrior of the universe, the super saiyan, a warrior said unbeatable, with the powers that would make him invincible. Vegeta , when was a kid, already had more power battle that most saiyans, even his father, King Vegeta, faithfully believing that if there were someone who could become the legendary warrior, it would be him.
We see Vegeta getting stronger and stronger, specially with his suicide attempt (done to increase his own power), and we see he fighting Frieza, imagining that he had already attained the status of super. Frieza is frightened at first, but after humiliate Vegeta, says that he shouldn’t have been worried about a silly legend, since the suppose super saiyan was that stage of Vegeta’s power, far inferior than his, with his last transformation.
After defeated and almost dying, the prince of the saiyans reveals how his home planet had been destroyed, how Frieza humiliated and cruelly exterminated the saiyan race. Vegeta dies and leaves the revenge of his race in the hands of the last pure-blood saiyan warrior: Kakarrot, the son of Bardock!
Krillin’s death in front of Goku made him get angry as never seen before. The anger and hatred led Goku to hit the status of super, transforming physically into a golden warrior, much more powerful than anything that has ever existed in the universe!
Until now, we haven’t say anything you don’t know, it was just a review. Now, I’ll start working on the Theory of the Super Saiyan.
Goku managed to transform because Krillin, his best friend, who couldn’t be resurrected again, died. The anger combined to his calm and pure personality enabled to Goku’s transformation. Vegeta, in the other side, hit the level of super for being exhausted and for hating himself at the moment. The prince of the saiyans was training in one planet when a meteor shower began. Vegeta had to protect his spaceship at all costs, but in one moment, one explosion hit him and throw him to the ground. Angry and exhausted of his efforts to surpass his rival, Goku, Vegeta aroused the super saiyan. As himself explained later, “Yes… I am calm and pure… pure evil!”.
With Gohan the transformation was really enforced. Goku tried to make him transform with thoughts of Frieza killing Piccolo and the others friends, but Gohan only transformed after Goku launch a powerful kamehameha on him, with power sufficient to get killed (Gohan told Goku to try killing him and if he couldn’t protect himself, then he would be a delay in the training and wouldn’t be useful in the fight against Cell).
Mirai Trunks, in his time-dimension devastated by the androids, transform after seeing his friend and mentor, Gohan, killed by the androids.
The doubt starts there. Goku and Vegeta always trained more and harder than their children and these ones had much less difficulty in the transition to transform into super.
A possible explanation is that the pure-blood saiyans, for being warriors, have accustomed to seeing horrific scenes of war, deaths, murderers, etc, even because the they were the invaders and the ones who caused the horrific scenes. This fact of getting used to terror made them “insensitive*”. What causes the transformation into super saiyan is the sensitivity.
*Broly was the only exception of insensitivity. Goku’s cry was enough to haunt him all his life.
With the saiyans half-human, such as Gohan and Mirai Trunks, by having the human side, which is very sentimental and emotional, was much more easy to transforms, because they have the traits of a race so emotional. Gohan’s transformation was more premature than Trunks’s because they lived in different worlds. Gohan, when was little kid was pampered, until he had his training with Piccolo, but still, Gohan has always valued life. Trunks lived in hell, with the androids destroying everything and everyone. The world experienced by Vegeta’s son influenced the same way as the battles of the Saiyans: Trunks was insensitive, only turning when his mentor and friend Mirai Gohan died.
With Goten and Trunks, which are still more exceptions, the probably reason for they reached the status of super is that they were too pampered and as everybody knows, pampered children irritate with any little thing. Goten, for example, got pissed off with the great demand of Chichi about their training and ended transforming.
About the reason for Pan not transforming, I say that I believe in the sexist theory that women saiyan can’t transform. Many theories could explain that, but I’ll simplify it. The possible reason is that the Super Saiyan is in the X chromosome, but to activate it, you must have the complement in the Y chromosome, then, only XY (men) saiyans can hit the status of super.
But of course, this all is just a theory, I’m just supposing things here.
That’s it! This is my vision about the transformations into super saiyan. If you have other theory, please, share with us!
Planet Namek 
