Your brain is a battlefield peppered with electrochemical explosions; a wet bundle of nerves, firing at each other within a glue-like soup. It does some things well and others poorly.
Not only do you think with your brain, you also use it to perceive: it's the primary mechanism by which you collect information about the world around you. It's a bit like the fox guarding the henhouse: the same entity that provides you with information is also telling you what it means.
In response, Josh Jeffryes posits:
My own analysis of decision making and marketing is that the kernel and the low-level software surrounding it (emotions) are the most important elements when making a decision or trying to convince someone of something. The rational mind just supplies the justification for what the primitive mind is going to decide regardless of reason.
This maps to my experience as well. If someone has strong feelings on a matter, then all the factual, logical argument in the world won't change their mind. We have the ability to filter such input carefully to support our core, emotional position.