Personal preference for a specific type of career:
For example - I want to become a teacher. It's very well possible that my interest will change.
However there are fundamental reasons behind my enjoyment for teaching, ones that have existed long before I knew anything about teaching. I think choosing a subject based on those fundamental interests is the best idea as they aren't as changing.
I reacted very negatively towards the duty comment. Because that's exactly what I had been brought up to do and consequently I'm suffering for the decision. I had a mathematical background, my interest is more in humanity. Technically I could have concluded that my duty was to work in the environmental agency or hazard risk management except I hated the work. I don't think that's a good direction to go down.
Discover core values.
That's it really.