Depression is a complex condition with a wide range of manifestations, from loss of interest to anxiety and low motivation. It's also a risk factor for other illnesses and can worsen existing medical conditions. For instance, the American Heart Association has added depression as a major risk factor for coronary artery disease.
In terms of neurobiology, depression can be linked to the overindulgence in highly dopaminergic activities, which can overwhelm the pleasure system and lead to anhedonia, a state where one can no longer feel pleasure. This is often seen in cases of addiction and can set the stage for depression.
Another aspect is the deficiency in the norepinephrine system, which is common in many forms of major depression. This system is responsible for our alertness and response to stress, and its deficiency can lead to symptoms of depression.
Lastly, depression can also be linked to certain habitual behaviors that are reflexive and hard to break. These behaviors can reinforce the depressive state and make it more difficult to overcome.