We've all heard it, we all know what it means, yet many of us are lacking in the application of the concept. Common excuses are "but _____ is good for me!", "I like _____," " _____ is healthy!", and "I don't have time for the other stuff because _____ takes up all of my extra time/stomach space/etc."
So here's an example of things people like to do too much of, but can be viewed as something that is beneficial:
-Training in a specific sport
-Eating something considered "healthy" or "good" for them
Just because you enjoy something, or it's good for you, doesn't mean you should let it occupy all of your free time. I'll start with a small example. Broccoli. Everyone knows broccoli is good for you. Whether or not you enjoy eating it is another story, but in general, broccoli is a pretty safe bet if it's part of a healthy diet. I know someone who only ate broccoli (and quite a bit of it, because "it's good for you!"), and did some serious gut bacteria altering as a result, which landed him in the hospital, after having constant abdominal pains. Determined cause of pain: too much broccoli messed up the balance of bacteria in his gut and his stomach didn't appreciate it. (*side note, you can look up a guy who actually turned ORANGE from eating too many carrots!!)
Another example: Working out. It's great to be active, work out, and find a sport that you enjoy. Staying active and physically fit is great for you. However, doing the same sport ALL the time is not. Sure, you might be physically fit, but there are many aspects of movement you aren't getting just by practicing in one sport. Since my gym is connected to a Brazilian Jiu Jitsu gym, we'll just use that as an example. BJJ is a fine sport and martial art. Many people have lost weight by training BJJ, and also become addicted to it (since it's so much fun). The thing is, BJJ is a very flexion-based sport, so you tend to miss out on all the extension you need to counterbalance that flexion (especially since regular life is also flexion-based). Because of this, I see many martial artists have "lazy ass" syndrome (where their glutes are inhibited, and they're using other muscles to compensate, causing back pain and other discomforts), amongst many other physical deficiencies.
Studying, reading, etc is great for the mind and improving oneself, but doing only that in your free time, you ignore the physical movement your body needs to be useful. I see this a lot, being in Asia. People start out at a very young age (around 2-3 y/o), trapped behind a desk, slouching, rounded shoulders, heads forward. All just to get ahead in their formal education. By the time they're in high school, their postures are terrible and habitual, and they don't realize the damage they're doing to themselves structurally until later in life when they get a job, everything hurts, and people have already accepted that it's "normal" to have compressed lumbar discs, bone spurs, and flat feet.....in their 30's and 40's. I'm not sure about you, but to me, 40 seems way too young to be dealing with those types of conditions, unless you've been in some type of accident or something.
These activities/foods are good for you, sure, but in moderation. There should be variety in your life, with whatever you do. Variety will get you much closer to being optimal and super awesome than repeating the same thing over and over. If you enjoy broccoli, have some, but also eat your carrots, bell peppers, and beets. You need more types of nutrients than what broccoli can offer. If you enjoy BJJ, train, but also cross train in something that requires extension, like proper weight training, or even ballet (it's also been proven that athletes who cross train perform better in their main sport). If you love to/must read a lot, by all means, please go right ahead, but take breaks to go outside and play physically for a bit. Maybe climb a tree or go for a hike or play a pickup game of basketball (fyi, kids learn better when they get to take physical play breaks, rather than jamming through learning stuff for extended amounts of time. So do adults). Make time for the "other" stuff, while still doing the things you love. Those other things will make you better at what you love to do anyway.