We have "emotional reservoirs" of different types. Some supply "energy", others supply "calm", "happiness", or "well-being".
If the reservoirs are full, we may maintain our energy, calm, happiness, or well-being, even in times of stress. If there is a drought (such as a bad day or week, or other varieties of stress), we maintain a healthy state, even though there is environmental adversity.
If the reservoirs are dry, we become dependent upon the immediate environmental circumstances: there may be energy or happiness, but only if daily events are going well.
The combination of a "dry reservoir" and a "bad day" could be intense symptoms: an emotional crash, lost temper, sometimes thoughts of suicide.
Various psychiatric and medical conditions lead to a "dry reservoir" condition. Depression itself is depleting. "Personality disorders" could be understood as a "reservoir" problem in some cases. Chronic pain conditions of any sort can be depleting. And chronic environmental adversity, of course (e.g. ongoing abuse, oppression, etc.) can keep a "structurally intact" reservoir constantly dry.
To run wild with this metaphor a bit, I suppose there are different varieties of reservoir problems:
1) the "leaky reservoir" : good experiences are not internalized, noticed, or remembered
2) the "too small reservoir": only recent events (over days or weeks) determine the fullness of the reservoir
3) the "blocked reservoir" : there is an abundant inner supply of positivity, but symptoms persist, and the reservoir seems inaccessible
I suppose therapeutically, this reservoir notion could be worked on in several ways:
1) learning ways to "fill one's reservoir" on an ongoing basis -- so that one becomes less dependent on the immediate situation for well-being
2) "reservoir maintenance" : repairing leaks or blockages--there may be ways to consciously maintain, notice, hold onto, positive experience, instead of allowing it too "leak away" or be inaccessible.
3) discovering a reservoir that was always there, but that lay outside of awareness (here's a kind of psychoanalytic idea--though more about uncovering something positive rather than uncovering a hidden problem).
I intend this reservoir idea as a broad life metaphor, but there are direct analogies to be made between the "reservoir metaphor" and neurophysiology. For example, if neurotransmitter reservoirs within neurons are depleted (literally) by a drug such as reserpine, a depressed state will ensue (examples such as this are strong elements of support for neurotransmitter-based hypotheses--such as the serotonin hypothesis-- about depression). One of the problems about neuropharmacologic theory, though, is that it may focus excessively on quantities (such as "reservoir volume", or more literally, "serotonin function") while failing to attend to structure (structural change in the brain must be achieved through thoughts, actions, and relationships, not merely through changing "reservoir levels").