What is the diathesis stress model of depression?
A person has to be both vulnerable to depression and encounter a stressful situation.
The diathesis-stress model is a theory to explain mental illness which says that in order to become mentally ill (i.e. to get depression, schizophrenia, etc) a person has to have a natural susceptibility to that disorder and a stressful situation to trigger them.
Neither natural susceptibility nor stress alone is enough to trigger the disorder
Diathesis-stress applied to depression would say that a depressed person would be predisposed to having depression (potentially genetically) and a stressful situation to push them over the edge, such as a relationship break-up or migration.
If a person is only vulnerable to depression they will not necessarily get it. In the same way, if someone who is not vulnerable encounters a stressful situation, they will not become depressed. Both factors need to be true and entwine themselves in order for the depressive effect.