[D]epression is characterized as a reaction to losses / continuous exposure to stresses that makes goals out of reach / unachievable... One becomes withdrawn from the situation and does not fight the stress, but flights from the stress by withdrawing in a cocoon. The loss of appetite and more sleep can be seen as behavioral counterparts of withdrawing or exhibiting a flight response to stress.
[M]ania is a reaction to a situation similar to depression -- when something is lost / is under threat of losing -- but this time, under stress, one fights and not flights -- thus one becomes energized to right the wrong and may become angry / irritable if the efforts to retain goals / valued entities are frustrated by [the] external world... the focus is preventive and the state is of scarcity.
Contrast this to a state of abundance when ones (life) goals have been met / are within reach. This apparent positive state of affairs may again give rise to different emotions / behavioral manifestations depending on whether one has approach or avoidance dominant reaction. If one approaches the more free time available after goal accomplishment as a boon that can be used to hone ones hobbies / find other meaning in life / build relationships etc and not as a threat (free time can be a threat) then one experiences positive emotion of happiness and behaviorally flourishes.
In contrast consider a similar person who has achieved everything in life... but given the fact that one is living in abundance is frightened or flights from the free time that has been made available. [T]hat person will be listless, will exhibit ennui or boredom and may even exhibit despair as he finds life meaningless. Thus behaviorally he would languish.
The key distinction between these two continua is that they are pre- and post- goal emotions. While working towards a given goal the extremes of the emotional spectrum will be depression and mania. After the goal is met, the extremes of the emotional spectrum will shift towards boredom and happiness.
One of the most well documented biases is that we tend to assume that our post-goal emotions will be further towards the happiness side of the spectrum than they really are (i.e., see here). Perhaps if people had more precise understandings of the distinction between pre and post goal emotions, this bias wouldn't be so common.
Guatam's model also makes clear predictions about the two different types of depression that so troubled Jonah Lehrer's article about the adaptiveness of depression. I like his model because I think that in the real world there are very few thresholds and everything is on a spectrum of some sort.