We have two problems today: 1) people being encouraged to follow unreasonable dreams, and 2) people having their reasonable dreams doubted by others.
Problem 1: Fantasies
First, the idea you can "be anything you want" or "do anything you want" if you "just believe in yourself" is utter self-esteem, fanciful nonsense. I'll never be an NBA player or a world class pianist. No amount of positive self talk or "rah-rah" is going to change that. A reasonable is something that pushes you beyond what you have ever done but not towards something you are incapable of doing.
We don't do ourselves or those we influence any favors by encouraging them to pursue an unrealistic for the sake of "feel good about yourself" or "follow your dreams". I once had an in-law who was a little better than average decent singer. She could sing pretty well. She had a nice voice. But she was not professional quality, nor did she possess the drive, discipline or commitment to be a professional singer. But the whole family patted her on the back and encouraged her for 20 years to pursue her "dream" while behind her back rolling their eyes knowing full well it could never happen.
Don't encourage someone in an unreasonable dream just because you don't want to hurt their feelings. On the other, you should listen to someone who advises YOU that YOU are pursuing something unreasonable. Doesn't mean they are right, but it should be something to an honest look at, especially if the person advising you is a true friend with no agenda.
Problem 2: Doubters
The second problem is those who doubt your reasonable dream. These doubters are usually the mildly lazy, the lack-of-vision folks, the jealous and the criticizers. A two word answer for this problem: ignore them.
If you have a reasonable dream to create a business, learn a skill, pursue a career or accomplish something, YOU WILL have doubters. Ignore them.
As whole people dream too small… they don't push themselves to bigger things settling rather for the scraps and crumbs that fall off life's table. Dream big reasonable dreams for your life. No, that's not a contradiction; it's a buffer against our generation of self-esteem, celebrity-worshipping fantasy that would have every kid a pro athlete or American Idol winner.
Dream BIG reasonable dreams, then ignore anyone who doubts you can achieve them.