For me I had way too much integrity to become a lawyer or a politician and I didn't like disecting animals in science class so being a doctor was out.
I wasn't musical at all so I couldn't carry a tune.
I Couldn't draw due to a hand injury at 1 year old so being an artist was not an option.
I did however have no problem disecting radios and modifying them to operate in bands that they weren't designed for along with building radios from scratch and designing my own antennas which led to Amateur Radio at a very early age.
I always wanted to work with electronics and robots as far back as I remember -- less so with computers because they weren't readily available when I was a young lad (although they later grew to be one of the loves of my life).
When I was poised to graduate from high school, I was torn between going to art college or an engineering course at the university. My mom said that I could always do art as a hobby, and that very few artists earned a decent living at it, but that engineering would give me a good enough living that I could afford to do art as a hobby. She was right as always (dammit :-)