There is no reason one should not realize the gains, I have held coins worth thousands of dollars until they went bust, I know it's probably cause they were shitcoins and story would have been different if it had been BTC/ETH but It's always good idea to take profit. When to sell? No one can tell you, best thing I have come across is DCA all the way out as Poker Player mentioned.
I know that we are going to make some profit but if we are just happy with what we have, then we are going to be unhappy when we see what we could have made, so keep holding is better. That's what I do and I have been happy ever since, it's just a smart way of making money by basically doing nothing but just holding bitcoin.
That's because you are holding BTC, holding only works for few prominent coins, not for all. Most alts never recover and you'll likely lose your profits if you keep holding them, it's better to secure profits in such case.