There are more theories than I care to list, and most of them are probably a factor.
The only thing we can say with true confidence is that mental illness increases suicide rates. In particular, the chronic anxiety and depressive disorders. Most of which are neurochemical in nature and thus fundamentally random.
While women are more likely to contemplate suicide, men are more likely to attempt it and far more likely to succeed.
And Japan's numbers are complete BS. See, in Japan, the culture doesn't like the idea of unsolved crimes, especially murder. So if they can't find an obvious suspect, they label the death a suicide. To keep their numbers nice. Victim blaming at its finest.
Also- a lack of sunlight might exacerbate other underlying conditions, making you more likely to see suicides in colder climates and during the winter months. But even those are a minor impact.