Imagine that. Simply saying "I know you can do it" could change someone's day. Or their life.
There are certainly occasions when you should tell someone they cannot do something. Novice skiers should not attempt black diamond ski runs. College students don't need encouragement to drink too much and have sport sex. People with empty bank accounts should not invest in pyramid schemes.
But it seems like in most circumstances there is nothing wrong with encouraging people to do something, anything. Even if it seems silly, beyond reach or sure to fail. If all that is at stake is pride, then why not encourage someone to chase a dream? The likelihood of good outcomes is higher than that of bad outcomes.
On the flipside, there is little good that can come of statements like "you don't want to be one of those people who say they are trying to write a novel." Or "there's something wrong with women who want to be attorneys." Both of those statements were said directly to me, albeit in the 1980s and 90s. But even with more than two decades in between, they still sit like little poisonous mushrooms growing in a dark corner of my mind.
Negative statements are sticky. Let's give them a rest and try the positive approach. Tell someone who needs encouragement that you know they can do it. That you have faith in them. You could change someone's life.