It can. Situation depression, often called reactive depression, refers to changes in mood and functioning that result from something happening such as work-related stress or relationship problems. Symptoms can be as severe is "major depression, " including poor sleep, change in appetite, loss of energy, even suicidal thoughts. If it is overwhelming, seek help.
It is not valid diag. It is not a valid diagnosis. Is life situation triggers major depressive disorder, it is still major depressive disorder regardless of the cause, and if should still be treated as medical illness. However, if symptoms are not strong enough to qualify for major depressive disorder, it should be diagnosed either as adjustment disorder with depressed mood or acute stress disorder.