Why do we have the International Regulations for Preventing Collisions at Sea (IRPCS, The Rule of the Road, "COLREGS" or collision regulations - call them whatever you wish to)?
When the 19th Century dawned, there were surprisingly, no official guidelines for avoiding collisions at sea, despite the fact that ever since ships first took to the sea, they have seemingly done their very best to avoid colliding with one another.
Why do ships collide?
Perhaps the best explanation we have found is as follows, whilst walking along a pavement on a busy street, it is inevitable that you will bump in to someone due to large number of people using the pavement having very little space to move around.
Similarly, ships at sea may bump in to one another if there are many ships close to one another and if they do not follow the rules sincerely. It is difficult to determine what the other ship's O.O.W. is going to do, as no one has control over his actions. As you decide to alter to Starboard, the other ship's O.O.W. may decide to alter his course to Port, until the inevitable occurs.