"Physical symptoms are common in depression, and, in fact, vague aches and pain are often the presenting symptoms of depression…physical pain and depression have a deeper biological connection than simple cause and effect; the neurotransmitters that influence both pain and mood are serotonin and norepinephrine. Dysregulation of these transmitters is linked to both depression and pain."
Madhukar H. Trivedi, M.D. “The Link Between Depression and Physical Symptoms.” Prim Care Companion J Clin Psychiatry. 2004; 6(suppl 1): 12–16. (web: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC486942/)