Whether it is true or not that in markets like Bitcoin (crypto) ; one person gains only when the other losses some thing. Suppose one person bought one Bitcoin for $5000 in January 2017 and sells in June 2017 for suppose $10000 he gains $5000 by selling. Now the purchaser in June 2017 @ $10000 sells it for $6000 in January 2018 and as a result he losses $4000. So it is clear from the example that the profit of seller in June which was $5000 includes the loss suffered by the person who bought it in June 2017 and sold in January 2018.
If there are some other reasons for this profit / loss factor, I request other members to please share their knowledge with the platform because my opinion may be on the basis of partial truth and there may be other factors responsible too.
This is somewhat true but not always and here's why. What if the person who bought Bitcoin at the price of $10000 before the price goes down to $6000 did not sell his/her Bitcoin and just decided to hold it because he/she believe that the price will go up soon? he/she probably lose almost half of his capital but what if the price goes up again? finally he/she gets a profit. And you will not lose a lot if you are actively checking the price and can get out early when the price goes down. Bitcoin is not originally designed to be an investment or asset but due to its volatility is high and only supply and demand determines its price, people are taking advantage of it.