Reality is different to dreams. What you think you want to do, might not be how you envisioned it when you get there. Or when you get older, you realize you aren't really the person you thought you'd be as an adult.
I was in IT. When I was a kid, I thought it'd be my dream industry. I still love the tech, but...
To get anywhere in IT it involves wearing a suit and brown-nosing people. The higher you get, the less it is about the tech and the more it becomes about business lunches, meetings, conformity and politics.
I thought I'd enjoy the suit and climbing the greasy pole (Why? Because it's there! </yesminister>). I thought I wanted to be some sort of business man.
After a little while, I eventually owned up and accepted the fact that I'm just too much of a laid back, open minded, non-conforming and generally chilled guy to stomach that sort of thing. And honest... waaaaaay too honest

.
I really wanted to be a yuppie so badly. I wanted to drink the kool aid, and be more "successful" than my blue collar parents. But alas - you just can't take the steel city out of the boy

.
So I said "F*** that noise!" and went and became a train driver

(well... technically I'm still in training for the next 9 - 12 months).
And might I say - I freaking love my job!