The waves of pain that had only lapped at me before now reared high up and washed over my head, pulling me under. I did not resurface.- Bella Swan, New Moon, Chapter 3, p.84.
While bipolar disorder has a strong genetic component, emotionally powerful events in one's life often come into play in unipolar clinical depression (aka major depression), one of the most common forms of debilitating mental health conditions.
A new study looked at fourteen pairs of identical twins raised together as children, one of whom experienced clinical depression and the other of whom did not. Romantic difficulties were fingered as the likey cause in half (7) of the cases. The other cases were attributed to single traumatic events (2); employment difficulties (1), a mixture of factors (2), and no clear reason at all (2).
The study was: Kendler, K., and Halberstadt, L., "The road not taken: life experiences in monozygotic twin pairs discordant for major depression.", Molecular Psychiatry (2012). DOI: 10.1038/mp.2012.55.