I'm a little confused, and this has been bothering me for a while. As the title suggests, I would like to know how a Saiyan becomes a ''Super Saiyan''.
What I do know is, (well I hope

) that things can cause the transformation, or trigger them. For example, when Krillin is killed at the hands of Frieza, and Goku transforms, in a rage. But, my question is, why does he transform then? Was Krillin's death, that severe emotionally, that it caused him to transform
then? For example, why didn't Goku transform when Krillin died for the first time in Dragon Ball, or even in some of the other times a character has died, and Goku had erupted in anger due to it?
Is it a case of how strong you are? As in, was Goku (In Dragon Ball) not experienced enough combat wise, or was he even not strong enough power level wise? When I think of this, it makes me think of the Original Super Saiyan. I tend to think the Saiyan race got stronger as time passed and as their race developed. I have no clue as to where I got this assumption from. In fact, I could be completely wrong (please do enlighten me if I am missing something

). It also leads me to think, if the transformation is based on your power level, then why have only a handful of Saiyans accomplished this in the Dragon Ball Z timeline? Why did the Original Super Saiyan transform, if he is weaker than Goku (using the idea that you transform due to your power level)?
I hope most of this makes sense. I'm sorry if it doesn't. If you don't understand something, tell me, and I will try to rephrase it.