The correlation of melancholia and mourning seems justified by the general picture of the two conditions. Moreover, the exciting causes due to environmental influences are, so far as we can discern them at all, the same for both conditions. Mourning is regularly the reaction to the loss of a loved person, or to the loss of some abstraction which has taken the place of one, such as one’s country, liberty, an ideal, and so on. In some people the same influences produce melancholia instead of mourning and we consequently suspect them of a pathological disposition. It is also well worth notice that, although mourning involves grave departures from the normal attitude to life, it never occurs to us to regard it as a pathological condition and to refer it to medical treatment. We rely on its being overcome after a certain lapse of time, and we look upon any interference with it as useless or even harmful.
The distinguishing mental features of melancholia are a profoundly painful dejection, cessation of interest in the outside world, loss of the capacity to love, inhibition of all activity, and a lowering of the self-regarding feelings to a degree that finds utterance in self-reproaches and self-revilings, and culminates in a delusional expectation of punishment. This picture becomes a little more intelligible when we consider that, with one exception, the same traits are met with in mourning. The disturbance of self-regard is absent in mourning; but otherwise the features are the same. Profound mourning, the reaction to the loss of someone who is loved, contains the same painful frame of mind, the same loss of interest in the outside world—in so far as it does not recall him—the same loss of capacity to adopt any new object of love (which would mean replacing him) and the same turning away from any activity that is not connected with thoughts of him. It is easy to see that this inhibition and circumscription of the ego is the expression of an exclusive devotion to mourning which leaves nothing over for other purposes or other interests. It is really only because we know so well how to explain it that this attitude does not seem to us pathological.
S. Freud, Mourning and Melancholia, 1917