I reject totally that MS is anything at all to do with personality.
As it happens, I am a bit of a worry-bunny, but I think a quick tour round the forums will show you there are just as many people who are (or were, before getting ill) pretty laid-back and happy-go-lucky.
My bet is that ostensibly high rates of depression in MSers are accounted for by common misdiagnosis. I had been previously diagnosed with depression, but strangely, no anti-depressant helped at all.
I now feel that what I accepted, at the time, must be depression, was in fact the fatigue of chronic undiagnosed illness (MS). I think it’s quite common for people to have MS for a number of years, before it begins to be suspected.
I think early, undiagnosed MS is quite likely to be mistaken for depression, or to cause depression. The person is finding life subtly more difficult, and feels generally cruddy, but has no understanding of why! That would be enough to depress anyone.
I don’t think it’s the other way round - that depressed people are more likely to get MS. I’m not sure any more if I was ever truly depressed, or if it was just a convenient label stuck on symptoms not otherwise explained. I feel that if “depression” hadn’t been such a popular diagnosis these days, I might have been examined for physical causes much sooner. I think I’d already had MS for years, before even being referred to Neurology.
But it’s a bit of a vicious circle, because once you have a mental health diagnosis, NON-psychological symptoms are much less likely to be taken seriously. When you go to the doctor saying: “This hurts, that hurts; I’m starting to worry it may be serious”, you may be told health concerns are a manifestation of your depression and anxiety.