That may be true for severe cases. But if you consider the checklists used to determine depression, it offers the results on a scale. For example, the Becks depression Inventory (BDI - II) suggests that the depression maybe either "minimal, mild, moderate or severe". Similarly the Burns Depression Checklist (BDC) tests identifies depression as either "mild depression, moderate depression, severe depression or extreme depression". Anxiety is also measured in a similar scale.
The "mild or moderate" segment of depression, or anxiety, is what is said to be a more common short-term occurrence amongst all of us. Most of us manage to often deal with this kind of "situational" depression / anxiety due to some stressful event (losing a job, death, breakup etc.), on our own. But long-term and / or severe / extreme depression is what often requires an outside intervention.
The "mild or moderate" segment of depression, or anxiety, is what is said to be a more common short-term occurrence amongst all of us. Most of us manage to often deal with this kind of "situational" depression / anxiety due to some stressful event (losing a job, death, breakup etc.), on our own. But long-term and / or severe / extreme depression is what often requires an outside intervention.