One thing that I find fantastic about psychiatry is that you don't consider the aspects of their illness or personality in isolation. It needs to be considered in the full context of their life - personal history, health, family history, genetics, support networks, coping skills, life stressors and so on and so forth. It really is a wonderful way to look at a person, and I love the way that it allows you to see them as a WHOLE person and not just a walking example of a DSM-IV category.
I have always been a sucker for a complex story, and psychiatry is really getting stories out of people for a living, so I suspect that as I get better at the interview and see more professionals do it, I will enjoy it more and more.
I think that to understand patients (and indeed everybody in your life) properly, you have to see them as multi-faceted people who have both positive/healthy and negative/pathological traits, and who have the capacity to change (some more than others) with work and guidance. Everybody has potential to live a more comfortable and happy life, and I think that is something worth striving for.