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.