Carbon monoxide is dangerous because it binds with hemoglobin in the blood.
Hemoglobin is made up of proteins that bind to iron atoms. The structure of the protein facilitates loose binding of oxygen. On other hand, Carbon monoxide binds very strongly to the iron in hemoglobin. Once carbon monoxide is bonded to hemoglobin, it is very difficult to release. This, eventually results in blood losing it its ability to transport oxygen. Hence, the person will suffocate. Due to this, CO is dangerous.