I remember reading about this in a book, 'The enigma of reason', basically it was saying that reasoning was exactly that, we decided and then we came up with a reason for what we had decided and usually not the other way around.

This is because, the 'reasoning' part of our brain came from evolution when we started to communicate with others, we needed to explain our behaviour.

Which is fascinating if you think of the implications of that. In the most part we think we are being logical, but in reality we are pattern matching/impulsive and using our reasoning/logic to come up for excuses for why we have chosen what we had already decided.

It explains a lot about the world and why it's so hard to reason with someone, we are assuming the decision came from reason in the first place, which when you look at such peoples choices, makes sense as it's clear it didn't.