“Judge a man by his questions rather than by his answers.”  - Volaire
Question, question, question. Everything. All the time. This is good advice, but choose your weapon wisely. 

Questions that begin 'Why' suffer from two problems. First, people tend to get defensive when faced with a why question because they assume it's accusatory. Secondly any answer is permissible no matter how obscure or vague. 

Why questions ask only for the respondent's personal truth with no demand for translation to an external meaning. Answers tend to result in a dead-end. Consider the following exchange...
Q. 'Why did you do that?'
A.  'Because I wanted to'
The questioner is now in danger of either getting no useful information or pushing for what will appear as a justification, e.g 'Why did you want to?'. The respondent will likely feel they are being poked and prodded and react accordingly.

Consider this second approach.
Q. 'What was the purpose of doing that?'
The respondent can no longer answer 'Because I wanted to' because it makes no sense for that question. Instead they are forced to answer in more detail. Perhaps...
A. 'Because it made me feel good at the time'.
This opens up the possibility of questioning forward from here with increasing specificity and value. The next question might be "What about it made you feel good?' and so on, and so on. Now there is increasing clarity and momentum to this exchange, leading to a better understanding for both parties.

When you want to get to the heart of the matter , ask 'what' not 'why'.