"I have never lost money in any buy.. You have to sell to finish the accounting
Upon buying you've exchanged the money for shares- once those shares depreciate can you claim that you have the same account value? Of course not- upon depreciating the value is gone whether you sell or not.
All you're losing when you sell is the chance at recouping the loss on those particular shares in the event the stock bounces- the value went out the window whenever the share price went down. While it certainly has a chance to regain the loss, the loss is still there even if your chose to hold.
Similar situation with other commodities. Say you pay $50K for a car. Car crashes and has no insurance, the car is now worth $5000. Are you going to hang onto that car with the belief that you "haven't lost until you sell"? No- you now have a $5000 car instead of a $50,000 car no matter what you do with it.
Market Data 
Markets 
AI
