Excellent reply. I see four general sources of anxiety: Organic disorder, psychological trauma, low self-esteem, and, like you said, stress.
I always throw "organic disorder" in because we don't know enough to rule out organic causation for many disorders and we may find we are wrongly treating some disorders with talk therapy when it will only respond to medication.
I put psychological trauma separate from stress because its not only magnitudes of order greater, but it also has a different mechanism being invoked. Like PTSD, it comes about because of repression as a response to experiencing something horrific - something that sets ones sense of mortality to the forefront.
Low self-esteem would be the most common cause of anxiety, by my guess, and it is usually located in a general area. People are more likely to be anxious about capabilities or lovability - it gets triggered with even more specific repressed fears - like someone anxious of public speaking my have the subconscious fear, "They will find out I'm a fraud." And that could be from someone who has driven themselves to be a foremost expert in an area, but in a sense, the little boy in him is afraid, because of the way he reacted to his fathers harsh demands. Low self esteem won't cause the anxiety, but will lie under the immediate cause.
Self-esteem fluctuates but not greatly or very rapidly. We built up whatever our average amount is for a long time. But stress varies quite a bit. We change jobs or try something new. We move to a new stage of life or go through a health trauma. We break up with a loved one. We see what may be a economic depression looming on the horizon. Even the holidays tend to stress. Lots of people can't shake a feeling of sadness during the holidays.
My general approach for someone with moderate anxiety or depressed mood, was to start with talk therapy and see if that was enough - just for a short period. If the mood isn't changing, then evaluating to see if meds would be a good addition. If that worked and the mood is improving, experiment with stopping the talk therapy for a while and using just the med for a period. After a while drop the meds and see if all is well. A good talk therapist may never know if meds would help or not, but will know when it is time to shift to different techniques in talk therapy.
Psychology is working from the base - the philosophy of psychology - towards the theories, and then the techniques, and finally to arrive at an understanding right down to the physiology level. Biological psychiatry is working with the chemical pathways, the physical structure, and moving towards the understanding or normal and pathological functioning and to associate that with behaviors and disorders. Eventually they will reach, adjustments will be made, and it will be all one contiguous, coherent science of the brain and mind. But that day is way way off. Right now there is a very big gap between where one leaves off and the other begins.