Depression is often caused by life sucking. When depression is amply explained by this model, you become depressed when both (a) the present is unenjoyable AND

Depression is often caused by life sucking. When depression is amply explained by this model, you become depressed when both (a) the present is unenjoyable AND (b) you have no hope that the future will be better. I know that this is sometimes the case because I've struggled with depression on-and-off and it tracks 1:1 with my physical health and ability to live a normal, enjoyable life.