The professional experts are still looking for answers. They say it's a combination of environment, genetics, stress, etc. Mood disorders and schizophrenia (up until 40) usually tend to show up in adolescence and something usually triggers it. I think that's why it's important to have good Mental Health Programs at Colleges, Military, etc.

With cancer they say there's a latent period which can be very long (+10 yrs) and then it shows itself.