G. Mark’s Answer
Desire. I always tell my students that folks tend to be good at what they like to do, and they tend to like to do what they're good at. This lucky synergy makes for happy, successful engineers. Another nice thing about engineering is the tremendous variety of jobs and applications. It's unlikely you'll never find a job at it that you like. You may find a particular position boring or too stressful, but the job market for engineering is very large. My advice is to take classes and internships in a range of skills and take the one that suits you best rather than deciding on an occupation or position first.