Can you explain the emotional problems with MS?

I just found out my brother has multiple sclerosis and for the most part I understand everything. A year or two ago he was diagnosed with being bi-polar, turns out he’s not… It’s was just the MS. This is the only part I don’t understand and I can’t find any websites that touch on this part of it. I know depression is one of the symptoms but why? I think I just need someone to dumb it up for me, but everything else I think I have a pretty good grasp on.