...and then he's going to avenge his companions and leave Dorthonion.
But the reason we want to show his grief, to show him conflicted about the decision to leave...is so that we can show his healing in Doriath. He isn't just physically worn out and now in a place of safety with enough food. He's met Lúthien, and she heals his grief and allows him to overcome that guilt and experience joy in life again. If we don't show the hurt that he has suffered, we won't be able to depict his healing from it.
I was not at all talking about grief. Grief he feels for sure. One reason why he stays for years in the texts.
Still the mechanism to make him leave immidietaly - as after our change - if he was the one who has insisted on staying before makes no sense if we focus on his grief.
Grief is debilitating, anger is motivating - but he has spend his anger already on his revenge.
There must be a strong motivation to "pull" him out of Dothonion - at if it is strong enough to make him risk passing Nan Dungortheb now why was it not present before - and how comes he suddenly abandoning his previous strong motivation to stay?
This seems random and capricious - not attitudes we want to associate with Beren?
It could be explained but:
-this is convoluded as such it would take time to explain
-delving so much into his incoherences makes him less defined and attractive as a hero
-he is alone,so he has no way to explain himself to anybody on screen
the purpose of making him leave immediately was to simplify the story and to focus more on B&L - this would be contadicted by making Beren's choice difficult to get without a lengthy explaination
All of this is avoided by making him wanting to leave for some time now feeling that his fate calls him to be somewhere else.
Edit: making of of his companion being not dead yet when Beren arrives and making him commanding Beren to leave as his last death wish would work, but that would seem very cheesy as this has been done so many times before in movies...
Being through Nan Dungortheb is taumatic and poisonous enough in itself to be in need of heailing when Beren meets Luthien.
Being heavely impaired already when Beren enters Nan Dungortheb makes it unbelievable that he is able make it through this evil place at all.