I think its all in your mind... if you tell yourself that putting CLEAR EYES into your eyes will lessen or kill your high, and if you honestly believe that in your mind, then that is what will happen... but if in your mind you were to honestly believe that putting it into your eyes would open p your eyes (litterally) to new and wonderful things and increase your high, then that is what would happen too.

Another theory is that most people that believe it kills their high because they are putting it in when they are 1) not going to smoke anymore weed and therefore not maintain their high anymore anyway and 2) putting it in because they have to go somewhere where red eyes might not be aceptable and act straight, so its not the clear eyes doing it, its you stopping smoking weed and your environment changing.

I could go with either theory.