Why carbon monoxide is poisonous for us and why carbon dioxide is not?

CO is a highly poisonous gas. Its poisonous because it reduces the oxygen carrying capacity of blood. The mode of action and its consequences are explained as follows :

During respiration, oxygen molecule enters the lung and is then carried by red blood cells where it binds to haemoglobin, forming a complex known as oxyhaemoglobin. If carbon monoxide enters the lung, then it reacts with haemoglobin to form a very stable complex carboxyhaemoglobin (which is 300 times more stable than oxyhaemoglobin complex). It prevents haemoglobin from carrying blood, which results in suffocation, leading to death.