>>533525185
>Because it's cultural common knowledge in Asia that's shared by almost all settings with Ki / Supernatural Martial Art genre.
DB still explains things that are common knowledge, such as "You're wasting too much movement" and "You must have a calm heart", all of this because Goku himself wasn't aware of them, so having someone explain to him you mustn't lose control of your emotions for better ki control would still make sense. Or they could also have Goku himself explain that to Gohan, since he very clearly struggled with his emotions.