Question: How do abnormalities in brain chemicals (such as serotonin and norepinephrine) cause depression?
Answer: Well, there is evidence from both animal studies and studies in people that the people who suffer from depression have abnormalities in nerve transmission in the brain in both the serotonin and norepinephrine systems.
Now, the medications we prescribe to people to help depression -- the antidepressants -- actually work by improving norepinephrine and serotonin transmission. They don't actually give the body anything the body doesn't already have. What the antidepressants actually do is simply slow the breakdown of norepinephrine and serotonin in the brain, so the body effectively has more available to conduct nerve transmission in the brain.
The other thing is is that we now know that it's more complicated than simply norepinephrine and serotonin; there have to be other hormones or proteins or neurotransmitters involved, and it's a very hot area of research for the hope being to develop better antidepressants and more effective antidepressants in the future.