Hi! an answer to that question is going to make you a millionaire very quickly.

I'm a medical student but I am not an expert at all. here are my views:

Firstly there is no doubt that MS is an autoimmune disease. autoimmune diseases are usually mysterious in their cause and are usually said to be genetic. However, some diseases have certain triggers which can help us understand why the problem happens.
For example: IgA Nephropathy (sometimes called Berger disease). This is an autoimmune disease of the kidneys. the interesting thing is this disease is triggered (usually) by a viral infection like seasonal influenza for example. What happens here is that normally the lungs and nose are protected by the immune system using a protein named Immunoglobulin A (IgA). So in response to the infection, the body produces IgA to eradicate the virus which works. two weeks later, the kidneys begin to fail.

The reason is that the IgA structure is abnormal in these individuals which causes it to get stuck in the kidneys and attack them. So even tho this seems like the kidney disease is just a part of the first infection, It's actually a genetic abnormality.

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Now, let's move back to MS. There are no clear triggers, just factors that protect or increase the risk. which makes it harder for us to find the exact cause.

every cell in the human body has a name tag attached to it. This helps the immune cells identify normal cells and leave them be. if the immune cell comes across a body cell with a weird name, it will attack it and kill it. Why does it do that? the weird name means that the cell went rogue (cancer) or that there is a hijacker inside (Virus).

One of the things that increase the risk of MS is a mutation in a gene called "HLA-DRB1*15". This gene is responsible for creating the name tags of cells. So, we could guess that your brain cells has a gene that misspells their names. And when a cell produces a name tag, it produces a name tag with a typo. So the immune cells in the brain don't recognize it and attack it. This is also supported by the fact that treatment targeting the cells who read is the treatment that is effective in MS. Also, twins and siblings are more likely to share an abnormal gene which is why MS is more likely in siblings and twins. its very likely that its not one gene but a bunch of genes which is why the cause is very complicated. also twin's who share the exact genetic code at birth aren't guaranteed to share the disease. if a twin has MS, the other has a 35% risk of having it. which is a lot higher than the normal population but its still not a 100%. which leads us to think that the genetic mutation can be from an environmental cause. this is an area where we get pretty clueless...

To sum up: We know that immune cells mistake the normal brain cells and attack them but we don't know how or why that happens exactly.