Re-reading a book feels magical: the book didn't change, but you always see something new through each re-read. The book didn't change, only you changed. You are older and wiser (hopefully).
Your reading purpose also changed. You may only want to have fun on the first read, but from the second read, you want more. You want to spot the details that you missed. You want to study more carefully the characters, the stories, and its messages. Seeing all this extra stuff is harder if you only read for fun.
When I was a kid, all I wanted to do was play video games (who doesn't?). Most of what I used to think about was games and how I could play more (which reduced the time I thought about my studies/friends/family). These thoughts also oriented my behaviors to optimize that desire: the more I wanted to play games, the more I thought about them, and the more I actually played.
But I play less when I grow up. I think about games less, not because I want it less, but because I want other things more. As we grow up, we think less about what we used to think because now we can see how big the world is and how many things are waiting for us to try. Being kids, all we wanted was to be an X. X could be a "princess," "hero," "astronaut," or "president." But then we wanted more; we also wanted Y, Z, and W. That doesn't mean we don't want X anymore; X still looks appealing; it's just that we want to have all the Y, Z, and W more than to just have X.
What also changed is that although I could easily see how much games impacted me back then, it's harder for me to see what is impacting me now because 1) there are so many more things that can impact me, and 2) since there are more of them, the impact of each of them is less visible. Seemingly small and trivial, they affect our thinking more than we thought.
Usually, they stay quiet in the back of our minds. When the right time comes, they pop up in our heads, occupy our minds, and become what we think about (aka daydreaming). During our waking hours, thoughts keep bubbling up, making us ponder, disappear, and leave space for the next.
They can be the things that we have to face soon (like a test), the questions that we can find answers (why are we here), the questions that we can never have answers (what if we did things differently), or the questions that we already have the answers but don't contend with yet (a cooled-off argument), ...
Although most daydreamings serve no purpose, there are moments when they can be productive. One example is how we can actually be better at something just by thinking about it.
Before I became interested in cooking, my train of thought looked like this: What should I have for tonight? -> Maybe X -> Let's order it.
But after cooking got my attention, my train of thought changed: What should I cook for tonight? -> Maybe I can cook X -> But I already cooked X 3 days ago -> Maybe I can twist X a little bit, or I can try to make Y for the first time.
The result of that daydreaming session is that I will either cook X in a new way or, even better, try the completely new Y.
Let's have another example: suppose you not only like to cook but also want to challenge yourself. In that case, your train of thought can be: What should I cook for tonight? -> Maybe I can cook X -> But I already cooked X -> But I can make it better -> How? -> Maybe I can try that new trick -> Or use better ingredients?
Because you subconsciously strive to push yourself at each stop, your subconscious guides your train to the next level, giving you a higher chance of creating better dishes than others.
There are also times when my interests impact me more subtly. For example, another effect that cooking had on me is that it's more notable to me now.
After I started cooking, I wondered why everyone talked about cooking more. Was there a trend or a scandal about a restaurant? Was there a new trendy food? Then I realized, no, people didn't talk more about cooking; I just paid more attention to this topic. The frequency of cooking being mentioned stayed the same, but because I didn't care about cooking before, every time someone mentioned it, I zoned out. I contributed less to the conversations when the topic was about cooking. I daydreamed about things I cared about, waiting for the topic to change. I ignored every piece of information about cooking that was floating in the air. You can actually guard yourself from knowledge if you really don't give a shit about something.
This illusion has its own name: the frequency illusion.
The frequency illusion (also known as the Baader–Meinhof phenomenon) is a cognitive bias in which a person notices a specific concept, word, or product more frequently after recently becoming aware of it.
It's like buying a new pair of shoes and suddenly seeing everyone else wearing them.
Another way they can affect us, especially when we are obsessed with them, is by making us link everything back to them.
If you read enough non-fiction books, especially those that give advice, you will see this common theme: life is about X (with X as the object the book writes about). A book that talks about the importance of bravery? Then there's a high chance the book's theme is "Life is about bravery. Like, really, everything is related to bravery, and you should be brave". A book about negotiation? "Life is about negotiation. To live is to bargain". The more we are obsessed with something, the more we link everything back to it.
In recent months, along with learning to cook, I have also tried to build some desktop applications. At first, I was afraid that I wouldn't have any ideas to start with. And it was true: I had none in the first few weeks. But as time went by, I began to see some ideas.
Before building apps, whenever I thought about something, my train of thoughts looked like: X -> Something related to X, like X1 -> Something related to X1, like X2, or -> Something totally different, like J or W.
But after I wanted it, my mind kept linking whatever my current thought was to the "apps," like: X -> Wait, can I make an application out of X?
It's like my brain unconsciously relentlessly makes all combinations possible between my current thoughts and "apps", which indeed gave me more ideas.
It's just like how Fujiko Fujio discovered Doraemon:
Doraemon was originally conceived by Hiroshi Fujimoto following a series of three events. Firstly, when searching for ideas for a new manga, he wished that a machine existed that would come up with ideas for him. Secondly, he tripped over his daughter's toy (which was a roly-poly toy). Thirdly, he heard cats fighting in his neighborhood.
Seeing roly-poly toys and hearing cats fighting isn't rare to Fujimoto, but because that day he was also obsessed with manga ideas, he linked "roly-poly toy" + "cat" + "manga," which birthed one of the most loved fictional characters: Doraemon.
However, after building some apps for 3 months, I got bored. I switched to writing, which made me no longer want to find application ideas but to find writing ideas. What's funny was that I suddenly stopped seeing ideas for apps (which hadn't happened in the previous 3 months), and began to see ideas for my posts (which had never happened before). I stopped linking "Can I make an application out of this?" to any thoughts of mine and started to bond my thoughts to "Can I explore this topic more and make a post out of it? Is it that interesting to me?".
This also made me wonder: some individuals can be so creative because they are naturally gifted, or is their drive to find ideas simply stronger? Maybe it's both.
And finally, your interests also affect you without your notice. Each decision we make and each belief we have are being built not only by what we remember, but also by what we don't. Desires, beliefs, values, and fears don't materialize out of nowhere. They're either developed over time by our internal consciousness as observations and life experience pour in.
"Subconscious is motivated by emotion, right? Not reason. We need to find a way to translate this into an emotional concept." – Cobb, Inception
So yes, choosing things to care about is essential. Although some people can concentrate and detach their minds from their personal interests, most of us can't. Our consciousness is influenced by almost anything we are exposed to.
You can never be sure how big a thing can impact you. Although its only usual impact is making us daydream about it, sometimes, a small idea planted in the right place at the right time can change your life.
Since it's hard to get your mind out of something you're already too committed to, the best thing you can do is be careful of whatever seeds can get into your mind. "Prevention is better than cure" applies not only to your body but also to your mind.
"So I decided to search for it. I went deep into the recess of her mind and found that secret place. And I broke in and I planted an idea. A simple little idea that would change." – Cobb, Inception