The causes of Guillain Barre Syndrome (sometimes referred to as Guillain-Barre, or GBS) are an area of active neurological research and is still not fully understood. There is sometimes even conflicting information about the syndrome available to patients, therefore the information here is taken from reliable sources and presented with care. You can see the sources used listed at the bottom of the article.
Essentially Guillain-Barre syndrome is where the body's own defence mechanisms turn on themselves. The antibodies which usually only attack foreign bodies, such as a virus or bacteria, now attack the protective sheath, called the myelin sheath, around the nerve cells. Sometimes the body can even attack the nerves themselves. One thing that is known about the causes of Guillain Barre Syndrome is that is cannot be transmitted from person to person, nor is it genetic. You do not catch it from someone else (although the associated viral or bacterial infection could be) nor are you more likely to have it because a relative has.
The sheath that is around the nerves allow the signals contained within to traverse long distances (such as from your brain to your leg) and to travel efficiently. When this sheath is compromised so is the body's ability to talk to its muscles and skin, or to hear back from them, causing the temporary numbness, tingling and paralysis involved with Guillain Barre syndrome. The numbness common to Guillain-Barre is due to the inability of the nerve cells to relay the signals the body is receiving to the brain. Similarly pain, tingling or other odd sensations are caused by the confusion of these signals. The more serious (but thankfully not permanent) paralysis is caused by messages being disrupted in the other direction, from the brain to the body. Without these messages getting through the brain temporarily loses control of the muscles. Because the longer the nerve cell the more disruption that is caused, the extremities of the body, the arms and legs, are worse affected.
It is unclear what precisely causes the body to attack itself in this way, but it is believed to involve a prior bacterial or viral infection that happens within a few weeks. Research shows that sixty percent of all cases have had such an infection a few weeks before developing Guillain Barre Syndrome.
The infection could be as simple as food poisoning or something more serious such as HIV. It is uncertain what about the virus or bacterial infection causes the body to change its behaviour so, or even what happens to the body itself. One theory is the the virus confuses the antibodies into thinking more things are attacking than there really are, causing them to be indiscriminate with their attacks, and in this case causing damage to the nerve cell. Another theory is the the nerve cells are changed somehow to be seen by the body as foreign, and in need of destruction.
It is worth noting that despite historical claims to the contrary there is no evidence linking vaccinations with Guillain-Barre syndrome.