Dreams

Why is it that some people’s dreams come true & others don’t. For example some people say I’ve always wanted to be a doctor since I was 5 where as they are doctors & others are like I’ve always wanted to be a designer since I was 5 & are not in totally different careers. Is it because I know some people have kids early and need to put a hold on their life or other change their minds & others just aren’t fortunate enough. Kind of making me scared for my future because right now I am going through a major change in college. Did anyone ever have a dream to be something & successfully became it?