Don't get carried away with the number "5" in five why.
Although you can usually get to the true root cause by going through the exercise of asking why and determining if it is the final why in five iterations, it may actually take more or less than five.
Don't get tunnel thought on the number five. It takes as many as it takes, whether it take two or eight, the concept of five why still can be applied.
I always wondered about the 5-why thing. If a person isn't smart enough to know when he's found the answer (and must be prompted to keep asking "why?"), how can he be smart enough to know when to stop at all? It doesn't make sense on any level.