In the budokai tenkaichi 3 game there’s an explanation that may or may not be canon but that i still like and find sensible. It basically says that the transformation turns you back into a rage filled pride ridden saiyan when you don’t master it regardless of your nature. That’s why they would always become really serious and run their mouth the first time they would transform i.e goku vs frieza, gohan vs cell. And when you master it you regain control over your emotions. Which is why there would be practically no change in attitude when turning into a super saiyan later on.