For me, why you should never sell your bitcoin ever, its because of the increase or decreases the price of it. We all know that bitcoins amount is fluctuating too much that it will be complicated to predict its amount. Maybe the second reason is the bitcoins limitation, bitcoin is limited in 21 million, and mines already generated almost of 80-85% of its total volume so in the future maybe bitcoin become a very expensive and rare cryptocurrency. 
First of all, the nature of cryptocurrency like Bitcoin and a rare asset like Gold are different. Many people thought the limited supply of Bitcoin is the primary reason make the price of Bitcoin increase. However, it is just a "reason" for new investor want to invest money into this field.