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.
it is not always like that as profit to one investor will be the loss to another.There are situations like crash or all time high,which gives equal opportunities to everyone and it's just one's smartness how you handle this opportunity.But Opportunities comes to brave one and who are ready to take risks.One who wants to trade safe has very limited opportunity as well as returns are also less.