It's a myth. I mean Rage. I mean near death experiences and training. I mean otherworld training. I mean golden oozaru and then maintain control. No scratch that, I mean ritual with other saiyans of pure heart. Oh nevermind, God ki through training, and then add ssj for color change and buffs. Add some kaio-ken(goku exclusive). Or eat a spirit bomb implosion to speedrun attaining an even godlier form?(goku exclusive?). Go even further blue(vegeta anime exclusive?). Go even further blue plus some red and opposite of mui goku(vegeta only?). Tingle on your back. One in every universe green boy ssj. Old man 24 hour ritual. Piccolo dying(Gohan exclusive). Krillin dying( one time use). Let's not forget then can also fuck with their body mass and have useless muscular forms.

Probably missing some, and that list was poorly worded, like the reason for new forms. What isn't a reason for becoming a super saiyan is easier to answer lol.