The best part of working in the psychology field is the sense of fulfillment that we are helping people live better lives. I am an industrial-organizational psychologist and work to help individuals have better experiences at work (where we spend a significant portion of our lives). It is extremely rewarding to know that you empower someone to become successful at work or in their career through self-growth and development. There are few fields that can impact a person's life in such a manner.
Currently, I'm fascinated by personality disorders. Being able to see the manifestation of a diagnosis walking around in real time as you become aware of dysfunctional behaviors that are presenting themselves. Basically it seems like being able to make sense of something that almost seems to be invisible. What you will experience are behaviors which don't seem to be quite right. Then with your knowledge in psychology you can possibly begin to see the manifestation of a diagnosis. This will be especially helpful when you get people coming to you as a professional saying, "I'm constantly doing the same thing over and over and it gets me into trouble but I don't understand why" or you will encounter, say, a child who is angry a lot but doesn't know why. In your work when you begin to provide help and relief to minds that are confused, in anguish, etc. it will hopefully afford you a sense of pride, accomplishment, and satisfaction that you will be truly making a difference in your efforts to make this a better world to live in.