@matigo I don’t think there’s one single reason. For some people it does seem to have its origins in something that’s problematic in their life — stress, loneliness, fear… For others it’s more biochemical. Hormones, inflammation. There are probably lots of possibilities.
I know that in my case, it’s not about unhappiness. Sometimes I just have to curl up in the corner and wait for the worst of it to subside. Sometimes there are things I can do to help. Reducing inflammation helps; getting enough sleep; eating well (lots of vegetables, not too much sugar); doing analogue things — painting, writing, cooking, walking, yoga, reading full length books… and not too much time on digital things.