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...