From my experience, here are 4 reasons I discovered:
1/ Lack of Discipline— a) Not sticking to one strategy, b) not respecting your trading rules, c) not sticking to your plan.
2/ Lack of knowledge—Inadequate understanding of the market, trading strategy, financial instruments, and fundamentals can lead to poor decision-making.
3/ Not having a mechanical trading system—This leads to emotional trading. Decisions driven by emotions like fear and greed rather than rational analysis result in significant losses.
4/ Overtrading—Taking many trades will affect your decision-making ability and lead to poor decisions.
If there are other reasons why traders lose money, share them with me.
Even though all of those are true. But even some of the experienced traders out there are still going to have a bad day even though they don't have all of those. I don't think you can completely free from losses even though you "master" the technique from fundamentals and advanced stuff, you can still get thrown away by unexpected events thanks to the nature of cryptocurrency. Just do your best.