Person A loves to play computer games. They spend all their free time playing games, which means less social time, no after-school activities, less time studying, and lower grades, but none of these things bother the person because they're just happy doing what they're passionate about.
Person B lost someone they love. They feel bad almost all the time. They still get good grades in school, keep up with their activities, and are nice to everyone, but inside, they feel empty and broken.
If you had to pick which person has a problem, who would you choose? Person B is the obvious answer, but an external screening of these two people will actually say that Person A has a problem and Person B is okay. Why? Because the psychological definition of something being a problem is that it affects someone's ability to function, not that it makes them feel bad. You can see from the example above that how someone is "functioning" (in this case, doing well in school) does not always relate to whether or not something is wrong.
A person's grades could drop because they are depressed or are going through a difficult time. A person's grades could also drop because they joined the gymnastics team and are having the time of their life. It's hard to use external changes to judge whether someone has a problem.
Personally, I want to feel good just for the sake of feeling good, not because it helps me to accomplish anything. Feeling bad is a problem in and of itself for me - not because it prevents me from functioning. I went to a school where people talked about how little sleep they were able to function on, but I was never interested in functioning. My only interest was to sleep enough to feel good, regardless of what I was going to do. For some people, feeling good is a means to being able to function, but that's not true for everyone and certainly not for me.
The best way to figure out whether or not something is a problem for someone is to just ask, "Do you feel bad?" or "Is this causing a problem for you?" Don't ignore someone's problem because they're still functioning fine, and don't diagnose someone with a problem just because they're not adhering to expectations. It goes back to understanding that Person B has a problem and Person A doesn't.