Actions and Reactions
Human nature is to seek a connection between actions and their resulting outcomes (reactions). The attempt to find this connection is where learning begins. These connections can be very small and obvious, such as when you prick a balloon with a pin, it bursts. Or it can be the consequence of an action, for example, if you catch someone lying, you will trust that person less in the future. One of the keys to human development is to find and internalize as many of these connections as possible. Everything we do every day is based on previously understood actions and their resulting reactions.
One huge advantage of observing these reactions is that we can change something based on them. If your action does not produce the desired reaction, then next time you can try changing the action and see if the reaction differs. For example, if you pass a soccer ball to a friend and they can't stop it in time, next time you will pass the ball more gently. This might cause a different reaction, and it might happen that you pass the ball too softly and it doesn't reach your friend, so next time you will try differently again, and so on.
When all these things come into poker, finding the connection between action and reaction (or in this case between decision and result) can be much harder. Every action taken can have a wide range of outcomes, some favorable, some unfavorable. All this makes the learning process much more difficult. Applying the same learning methodology in poker as in kicking a ball can cause many problems.
Some poker scenarios can be solved mathematically. For example, in limit hold'em it is very easy to prove that it is almost always correct to call one bet with a flush draw on the flop. Many situations in no-limit are much harder to solve mathematically, such as whether it is better to fold, call, or raise with 66 from middle position. In this situation, there are many other influencing factors, such as your chip stack, actions before you, who your poker opponents are, your image, and the ICM factor (if it's a tournament).
The main mistake beginners make when learning from trial and error is that after choosing an action and getting a negative result, we instinctively change the action to get a better result. You have to be careful because a good decision in poker can bring a negative result, and a bad decision can bring a positive result. For example, we raise pre-flop with AA and lose a big stack of chips when the opponent hits a set. Or, let's say, we call a pre-flop raise with K-9 and beat the opponent's A-K when the board comes K-9-6. These are just a few examples, but the principle is the same for many decisions you make at the poker table.
Just because you are a winning player does not mean you are playing well. And conversely, just because you are losing does not mean you are playing poorly.
It is very important for a poker player to understand this. Otherwise, you might stop changing bad decisions in the game and continue making them because they benefit short-term results. In such cases, the player with AA will play too cautiously next time, and the player with K-9 will reform their opinion and think that calling with K-9 is good.
Source of material: The Poker Mindset: Essential Attitudes for Poker Success