What causes neurotransmitters to become imbalanced?

What causes neurotransmitters to become imbalanced? What symptoms would I have?
Our body is wired in such as way that the control center rests with the brain. A sense, such as noise, smell, etc, enters our brain and is then interpreted by the brain as being friendly or not friendly. The brain then take actions by way of hormones to effect change. Neurotransmitters are the messengers that start the hormone release. There are many kinds of neurotransmitters. Some are designed to stimulate us, others to relax us. Our bodies keep all these in perfect balance 24/7. When excess stress is present, imbalance begins to occur as the body is unable to balance itself. Certain neurotransmitters become more prevalent while others become deficient. Symptoms varies depends on the degree of imbalance both in absolute and relative basis.