The theory of 5 Why was developed by Sakichi Toyoda and was later used within Toyota Motor Corporation during the evolution of their manufacturing methodologies. Base of these theory is to derive root cause of any challenge.
Lets have a look for an example given by wiki.
My car will not start. (the problem)
Why? – The battery is dead. (first why)
Why? – The alternator is not functioning. (second why)
Why? – The alternator belt has broken. (third why)
Why? – The alternator belt was well beyond its useful service life and has never been replaced. (fourth why)
Why? – I have not been maintaining my car according to the recommended service schedule. (fifth why, a root cause)
It seems, that we need to digg until 5th why to resolve error up to final level. If you stop by to 4th or 3rd why, it would not help you to solve it permanently. It helps to have 1.5 Degree move for six sigma too.