We should not discourage others from selling their bitcoins, but it is a good advice not to sell them because bitcoin is scarce and a valuable asset.
It's not about discouraging them from selling, but discouraging them from selling all/ to much of their their holdings cheap at once just to avoid any future regret.
There're some point an investor will reach and he/she will fell like he has enough stashed up already to start taking profit, the deal at this point is not to sell everything but gradual taking of profits and at the same time it doesn't stop investing just that it can be flexed unlike someone who's still accumulating/ in early accumulation stage.
Thus, it's not like investors cannot sell their holdings , selling it's the purpose of investment afterall
but they need to do it in a manner that's not going to affect the investment and will sustain the investment for a very long period of time...
There's a big difference between discouraging someone from selling their Bitcoin and discouraging them from selling at the wrong time, left for me am a big fan of long-term goals when it comes to Bitcoin and I won't encourage anyone to sell their Bitcoin regardless of anything but on the contrary if someone has held for a full circle and more, it's their choice to sell or sell some portion of it if they're wise enough. However I don't think it would be very wise for someone who's investment is not due for taking profits, I mean someone who's not held for about 4 years or more to sell their Bitcoin, it's not like I'll force them or stop them from doing that, it's their choice and moreover they have every right to do whatever they wish with their coin but the best I could do is to only advice them on the consequences of making the wrong decision when it comes to Bitcoin investment and also remind them of what they stand to gain if only they could have patience. Sometimes you don't need to be aggressive or force people to make certain decisions even though it's the right thing, your good manner of approach and how you handle situations calmly would force people to make the right decisions.