The thing about this for me was that I didn't choose it - it chose me.
I chose to get into programming, the rest was history and experience and trial/error. Basically, I naturally found that I gravitated towards security-related programming and code analysis. This drew me to go to security-related events, conferences, meetings, and also apply for security-related coding jobs.
I never really struggled with what I wanted to do in computer science. I love it all but my "why" is helping to keep other people and their data safe.