My other brother bought at ~$2700, sold half of that for I guess $3xxx during the crash from $5000 to $3000. I told him he was crazy and he absolutely should not have done that. And of course if he waited just a few months he could have sold at $10k or $15k of $19k. He put in so little money relative to what they have (he just put in $2500 to buy .89 BTC) that the risk of losing that a couple thousand dollars when you have a good career is really not a risk at all, but the upside if you hold is maybe you sell in a few years and could possibly make $50k or $100k or more (or he could have sold late last year and made over $10k even after taxes for a tiny 6 month investment). So why did he sell half his BTC, worried that he was going to lose all of a few hundred dollars, and took the out of gaining a few hundred dollars, pure emotion of "oh my god my investment is dropping i better sell!"
First rule of bitcoin: never listen to your emotions.
And of course I've violated that rule plenty of times, especially during my first bitcoin stint back in 2013/2014, and it pretty much always results in losing money or losing bitcoin.
We often act without being completely aware of what we are doing. Yes, it may sound a little strange, but it is in fact so. Most of the time we act on autopilot without thinking twice or even once, while our actions are based on our past experiences. Thereby we repeat our past over and over again. Our emotions play a major role in this because when acting in default mode, our actions are fully determined by what we feel, not by what we think or find rational or reasonable. As explained in the OP (or somewhere in the thread), thinking is a conscious effort driven by awareness, whereas behavior driven by emotions is the opposite of that.