What's the difference between "situational depression" and depression that’s not situational?
Depression is defined by a series of symptoms that involve changes in sleep, energy, concentration, appetite, etc. One does not need to have a reason to develop a depression, though if there is a particular event that may have precipitated it (for example, a loss or a separation), that may be relevant to the psychotherapy and understanding one's vulnerabilities to depression.
Not everybody who experiences a loss or a stressful event develops a depression. Modern concepts about clinical depression maintain that someone either is or isn't biologically vulnerable to developing the illness. Also, the illness can either be triggered by certain life stresses or may arise spontaneously for no obvious reasons.
Clinical depression is different from normal grief or sadness. Grief or sadness related to an event does not involve disruptions of sleep and appetite or thoughts of suicide. In fact, normal grief can become complicated by a depression, which in turn can interfere with the resolution of the grief. So, in other words, depression is a syndrome involving many components. Regardless of what may cause it (we don't really know), depression should be treated. When the syndrome involves strong physical symptoms (sleep, energy, concentration, not functioning, thoughts of suicide), medicines are especially likely to be helpful.