You're told to be realistic. This is often a good idea. You strive to be realistic and wear it as a badge of honor. When asked if you're an optimist or a pessimist, you take pride in responding, "Well actually, I'm a realist."
Subtext; "I see everything perfectly rationally with complete information and make the absolute best decisions and have the best judgment 100% of the time, you poor delusional optimists."
Being realistic is often the best way to make good decisions. Figure out what can go right, what can go wrong, what the risks are, and what each of their relative probabilities are. It's a great framework for thinking about most things.
But sometimes, if you want to achieve something great, you must be unrealistic. Not in an "I'm going to defy gravity" type of way. Rather, acknowledging the high risk of failure of a pursuit, the small chance of success, the immense obstacles along the way, the naysayers that will criticize you, and deciding to do it anyway. In order for this to make any sense at all, you must have a big and important enough reason for your endeavor.
In the improbable chance of success:
What would your life look like?
How would the world be better?
Whose lives would you impact?
"When something is important enough, you do it even if the odds are not in your favor."
- Elon musk
“Positive thinking won’t allow you to do anything, but it will allow you to do everything better than negative thinking will.”
- Zig Ziglar
"He who has a 'why' to live can bear almost any 'how.'"
- Viktor Frankl
"Here’s to the crazy ones. The misfits. The rebels. The troublemakers. The round pegs in the square holes. The ones who see things differently. They’re not fond of rules. And they have no respect for the status quo. You can quote them, disagree with them, glorify or vilify them. About the only thing you can’t do is ignore them. Because they change things. They push the human race forward. And while some may see them as the crazy ones, we see genius. Because the people who are crazy enough to think they can change the world, are the ones who do."
- Steve Jobs
Sometimes, accept a high risk of failure for an important enough calling. If you want to change the world, sometimes, be unrealistic.