There is much debate about what the cause of Multiple Sclerosis is. Some believe there is a possible genetic component. The myelin sheaths(fatty covering) on the nerve endings, or axons gets attacked by the body. This causes the neurons not to be able to conduct nerve impulses properly. When the body attacks itself, this is called an autoimmune disease. There could also be environmental factors that cause M.S. However, at this time, there is no clear consensus about its cause.
There is not really much to tell here, so far as I know. From what I know, scientists do not know what causes MS. They do believe that it is an autoimmune disease. They know that the body's immune system attacks its own myelin, which coats and protects nerves. But as to why the immune system does that, there is no clear answer.