It's hurtful to doubt people.
Kate is gym teacher, and one of her students wants to be a pro baseball player.
Kate wants to say something to the kid.
Kate tells the kid that if they keep working then it might happen.
Kate tells the kid that there's no chance that they make it to the majors.
The kid feels motivated to work even harder to be a baseball player.
The kid's feelings are hurt, and they're embarrassed in front of the class.