Many of our moods depend on our nervous system. Too much or too little of the chemicals that speed things up or slow things down and the whole thing can go out of kilter. The basis of many of our medications is to correct these imbalances.
Out of balance brain chemistry can cause depression, anxiety, eating disorders, sugar and carbohydrate cravings, weight gain, addictive personality, reckless/risky behaviour and of course bipolar syndrome.
http://www.healthcentral.com/slideshow/getting-to-know-your-5-essential-brain-chemicals
http://www.balancingbrainchemistry.co.uk/peter-smith/1/Brain-Chemistry-HOME.html