Regarding what makes me tick...

Someone once asked me what I wanted to do and the following conversation ensued:
I want to be an engineer.
I know that's what you want to be, but what do you want to do?
Work with tissues, cells, and DNA?
No. That's not what I'm asking. What is it you want to do? What motivates you?

So for the past few weeks I've been plagued by this conundrum. How will I ever be good at something if I don't know what it is that makes me passionate? I mean, there are things I enjoy doing, perhaps to the extent of me being enthusiastic about it. For example, if you have ever seen me sing or paint you may be able to attest to that, but what, in the real world, will drive me to do what I plan on doing for the rest of my life? To that simple and straightforward question I have no answer, but I fully intend on finding out.