(…) Our hearts also have muscles.
What happens if you don't cultivate mental strength? You won't be able to exert your mental power, you'll suffer from anxiety and depression, and your mind will age faster.
Growing old means losing motivation.
Then you get used to giving up.
To protect my easily hurt heart, I avoid people and increasingly hide in my room alone.
In that room, I cover myself with a blanket called 'depression' and fall into a deep sleep.
And then I try to reassure myself.
If you want to protect your easily injured heart, you must develop your heart muscles.
I need to radiate positive energy from my heart and increase my basal metabolic rate in life.
When you're mentally exhausted, everything becomes a hassle, everything feels scary and you just want to avoid it.
Just as we need to get regular checkups for our bodies, we also need to get regular checkups for our minds to maintain their health.
You should practice building your own mental muscles on a regular basis.
How much muscle mass do I have in my heart? Remember, cultivating "heart strength" allows you to radiate positive energy and increase your basal metabolic rate, which fuels relationships.
---From the "Preface"

Muscles are also called 'himsal' in Korean.
It's the fat that gives our body strength.
If we don't build muscle, our bodies won't be able to exert strength.
Our hearts also have muscles.
If you want to protect your easily injured heart, you must develop your heart muscles.
I need to radiate positive energy from my heart and increase my basal metabolic rate in life.
How much muscle mass do I have in my heart? Remember, cultivating "heart strength" allows you to radiate positive energy and increase your basal metabolic rate, which fuels relationships.

--- p.4

Your character is revealed in your 'refusal attitude'.
It's hypocritical to accept something because you don't have the time or money, but you find it hard to refuse, or because you're afraid of ruining the relationship.
People who often make unreasonable requests that make others feel burdened are self-centered and only care about their own interests.
It's okay to become 'very' distant from those people.
Unless you can happily accept, wisely declining is the way to save your relationship with the other person.

--- p.56

If a glass is full of water, what happens if you add more water? Of course, it will overflow.
Our hearts are the same.
When emotions like depression, anger, irritation, and disappointment overflow into my heart, I can't even take in a single joke from someone else.
There is no room for even a single seed of a little joke.
Some people are born with big, thick hearts, and some are born with small, thin hearts.
The smaller and thinner the heart of a person, the more it hurts.
Poisoned words and swords easily crack and break.
Emotions like sadness, depression, anger, and despair don't go away on their own.
Therefore, it needs to be emptied frequently.
You need to empty your mind often so that you can hold more positive emotions.

--- p.118

People who struggle and fail at their jobs have one thing in common: they make complex plans! People who make simple plans and think simply are more likely to make good decisions and find the right answers.
Even in science, simple theories ultimately win.
Simple theories are easy to test, and in precise science, it is easy to create mathematical models.
Complex theories are difficult to test and can lead research in the wrong direction.
Life is the same! Complicated thoughts and worries are a shortcut to ruining your life.
Simple thoughts beat complex ones.

--- p.144

After a breakup, you need to face it instead of running away so that you can become 'a complete person without you'.
Only by overcoming the pain of heartbreak and letting go of the past can you begin a new love.
As important as the beginning is clearing up the past.
I need to take good care of my painful emotions rather than avoid them after a breakup.
Even if it hurts and is painful the more you think about it, you must face it and not run away.
You should reflect on yourself and try to use the breakup as an opportunity to grow as a person.
For my happiness, we must live loving someone until we die.
Because I need to use this experience as an opportunity to love more maturely next time.
The most important thing is that you must try to find yourself as a whole without you.
You have to fill the void of having half your life suddenly gone.

--- p.168

People who are not greedy do not need to compare themselves to others, so they do not need to feel jealous of others' appearance, financial power, or social status, nor do they feel the need to feel inferior.
I don't have to be the best, and I don't have to care what other people think of me.
Just set goals that match your abilities, be satisfied, and develop healthy self-esteem.
Isn't true self-love the confidence to show yourself and others your true self? If you truly love yourself, you shouldn't compare yourself to others, accept your unique self, and avoid abusing yourself.

“It’s pretty enough!”
“I’ve worked really hard to survive until today.
“It’s because of you that I came here!”
Let's look in the mirror today and praise ourselves.
If you look closely, you'll see that I'm actually a pretty decent person.

--- p.301