Jobs are temporary things, often enticing on paper until you realize that as soon as you get comfortable in your position, it will change, your boss will change, your team will change or your organization will change. That is just the nature of business. Therefore if you go into a job excited by the position or the person you will be working for and not the work itself, you often set yourself up to be disappointed.
Your life's work on the other hand, are activities that you have natural talent for, which energize you and stimulate you and do not change no matter what "job" you happen to be in. I found this for myself when I began to think about my own life's work. I reflected back on all the things I have done in my career and I came to the realization that the core of my life's work is about transformation