Somewhere down the line, chance or Lady Luck got involved and you were offered a placement which just happened to mean that you had to build some scenery, for example, and found that that was what you were good at - or what you wanted to do. You may have been forced to give up your ambitions to be a designer because you were not the risk-taking type or because you were offered too much money as a prop-maker or scene-painter. Or maybe you just thought you were not good enough.
One of the mistakes being made in higher education here in the US is that most programs are teaching students that being a stagehand is somehow inferior to being a designer. I had the head of the design progam at a Major Theatre Conservatory tell me, "We don't teach stagehands here."
In truth, some people are happier (and more suited to being) craftspeople -- the people who realize the designs -- and we do these people a diservice by pushing them into being designers.