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 the feeling and I know that there are a lot of people who want to take profit but in reality if you keep your profit then you are going to end up making even more profit and thats a good thing. I believe that it would be important for people to make more and more profit because it makes them feel hyped about it and I think it should be important for them to get a love for bitcoin as well.
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.