In the Dragon Ball Series, it is noted that the transformation exposes the heart to impurity, which is the reason Goku cannot use the spirit bomb offensively. (While in SSJ) In the Movie Super Android 13, he uses Super Saiyan while charging the Spirit Bomb, but it exposes his heart to impurity and the energy is absorbed within him. (Which is what happens when a impure heart attempts the spirit bomb), but why exactly does the Super Saiyan Transformation expose the heart to impurity?
I'm not exactly sure if my answer is what you asking for, but Vegeta tells 19 that when they turn into super saiyan they tend to be more savage. For transforming for the first time you have to be very furious, and probably full of hatred. That might be why Super Saiyan expose the heart to impurity, since to be pure-hearted means to be a person with only good intentions