The reason why becoming a good investor or trader is so difficult is that it runs contrary to what we consider common sense, if an asset is rising in price the most natural thing to do is to assume this is going to keep being the case and we will want to become part of that bull movement, however the bull market is the moment to sell not to buy, and the bear market is the same, as many sell at the time when they should be looking to buy instead since the price of bitcoin is very low during that period.
This kind of coincidence is closely related to a person's psychology.
When you say why they sell when the price starts to fall and should not, it is because they are too panicked and do not have any analysis of the upcoming price.
and this also applies vice versa, where someone will start buying when the price rises and FOMO occurs.
This will indeed go against common sense and be a difficult thing for some new investors who enter crypto.
Having good basic knowledge and psychology is very necessary so that there are no mistakes or losses in trading, so that strategies can be applied properly.