The laws of physic and human biology.
I wanted to be a garbage collector when I was young. I outgrew that desire I am doing what I like, pretty much.
My laziness. My goal when I was younger was to be a doctor. Totally possible. But, I am in high school, and should be studying harder, but I am just too lazy.
My parents weren't multimillionaires, as was the case with Bill Gates and every other entrepreneur who could pursue their dreams only because they never had to earn a paycheck working for somebody else.
the believe tht it's somehow either chore or difficult - ultimately a lack of self believe, and personal faith i.e. fear
...let me count the ways