Another variable as to why it is near impossible to play perfect, is that you do not always know what your opponent has. If you win the pot w/out a showdown, you may think you played the hand perfectly but you may have been able to extract more money from him (No limit, especially), so even though you won, you did not play perfectly.
This illustrates the fallacy of results oriented thinking in poker. The guy who cried is being way too results oriented, in this regard.