Jordan Peterson has that balloon analogy... Human's psychology is like a balloon, and it has some thin areas, weak points, and when the pressure inside is rising, the balloon breaks where the weak point is and explodes, that's how mental illnesses (depression is, no doubt, a mental illness) occur. The origin of the reasons for having those weak areas in your mentality doesn't actually matter, whether it's child trauma, genetical issue or tough circumstances, they all break the same way.