You are not trying hard enough. You only try hard at what you like.
This is a refrain you hear in school all the time. Probably because it's true, that kids intuitively try hard at what they like. In early grades, this means boys are trying hard at recess. In later grades it means very few kids are trying their hardest at math. It also means that we intuitively know what it looks like when a kid is be focused on trying their hardest.
As a student, I really took the criticism to heart and I wondered what was wrong with me that I wasn't trying my hardest at everything. After all, I didn't feel lazy. I wrote in my journal every day of my life. I wrote poems and stories that no one even had to assign me. I read all the books in my grandmother's bookstore (it was small). Yet I still internalized the voice that kept saying, "You're not trying your hardest. You do only what you like."
In hindsight, it's amazing that it's a valid criticism to tell someone they only try hard when they are interested in something. Because in the corporate world, where people actually study productivity, it's taken as a given that people will not perform well when they are not interested. So why try to force kids to do what adults cannot manage?
On top of that, adults who have career problems rarely have the problem that they can't do what they don't like. They have the problem that they are doing what they don't like and they don't know what they like. Because school doesn't teach you to figure out what you like. School teaches you to squash your inclination to focus on what you like.
I don't actually think that our society believes that people should try hard when they aren't interested. But our society is wedded to the idea of school. School cannot make an affordable teacher-student ratio work without teaching kids a bunch of stuff they are not interested in. Which means that people have to vehemently defend the idea that you should work hard at what you don't like. Because if we didn't defend school as a valid institution, then we'd feel bad sending our kids there, and then we'd have to homeschool.
I know, because that's what happened to me.
I think kids not trying hard enough is the precursor to kids not living up to their potential. Which I have always thought is complete BS. If you are not living up to your potential, it usually means you are not living up to the expectations your parents had for you. Because people can just be who they are. We can't be something else. And who we are is who we are. If your parents tell you, over and over again, that you're not applying yourself, you're not trying hard enough, eventually that becomes, "you're not living up to your potential."
And that's one of the most damanging phrases to take into adult life. If you fear not living up to your potential you don't trust your instinct. You lose the gumption to craft your own path because you fear failure. If you fear not living up to your potential you live the life your parents invisioned for you.
This is not what you want for your kid. I know you don't want this because the most unhappy people I coach are people who feel they are not living up to their potential. And I spend the first half of the session explaining to them why they don't need to follow someone else's dreams.
Set standards for your kids that enable you to say, "I like that you found something you like doing." Let kids know the standard is that they need to find stuff they like and do it. The more you encourage that, the more likely they will find the thing they adore doing. And then they'll work really hard. Because we all work hard when we love what we're doing.