I think that, to an extent, our instincts and emotions are detrimental to rational decision making. Many instincts were developed for survival in a world far more primitive than the one we live in today. Most instinctual reactions to fear, danger, etc. are meant for physical threats, but today we see them manifesting in response to situations where they are no longer helpful (i.e., situations where adrenaline and physical reactions don't help), causing anxiety and poor decision-making.
But at the same time, things like hunger and thirst are very helpful, imo. If your body didn't tell you to eat, you'd have to worry about consciously remembering to eat and drink all the time. The fact that our body takes care of this leaves us more attention to devote to thought.