In the most general terms, the cause of reactive depression should be understood as an excessively high assessment by a person of those life circumstances that he does not have both at the present time and in the long term, and - i n parallel with this - an excessively low assessment of the circumstances that are present both now and in the future. the future. This raises an important question: what essential feature of a person is decisive in terms of the presence or absence of depression? If we consider reactive depression from an existentialist point of view, then its cause is the ability of a person to lose a sense of non-existence. The loss of this feeling arises under the influence of numerous life circumstances, under the influence of the idea of high significance and long-term duration of psychotraumatic circumstances. The article deals with the issue of the causes of reactive depression - a mild mental disorder not associated with pathological changes in the body. An assumption is made that in order to analyze the causes, it is necessary to turn to existential philosophy: this is the only philosophical direction that explores the role that a person’s understanding of his own non-existence plays in human existence, assessing this role positively.