Why didn’t Guts open up to Griffith before leaving like he did with Casca during the bonfire talk. He expressed himself so well to her but he blew it with Griffith. He didn’t even try to talk to Griffith. I never understood this, Griffith had always been communicative with Guts. Griffith was so open that the band of Hawks were jealous of the recognition and treatment Guts received. Do you think there is any in-universe hope for reconciliation? It was all a tragic miscommunication.
Basically, because telling Griffith why he's leaving would undermine his goal. It's a contradiction, as Guts suggests in his inner monologue that night.
To be Griffith's friend and equal you have to have your own dream you dedicate your life to, you can't be compelled by another man. But Guts leaves to find his own dream because hearing Griffith's words compels him to. If his goal is to be independent of Griffith just so he can be worthy of Griffith's friendship, and it is, that inherently means he's not worthy of his friendship. According to Griffith's dream philosophy, that makes Guts pathetic.
So if he explains to Griffith why he's leaving, he should fully expect Griffith to look down on him for it.
Guts also has major self-esteem issues stemming from his abusive childhood that make it extremely difficult for him to trust that someone loves him, and very easy for him to believe that they look down on him. Hence why all the evidence of Griffith's feelings for him is erased by one overheard speech which seems to confirm what he's already inclined to believe deep down.
(Here's a post that digs into that, mainly about why Guts leaves.)
So he also has no reason to think that Griffith will have a self-destructive breakdown when he leaves - I mean, when he comes back he even spends three days in denial before he accepts that it happened. Guts doesn't think Griffith will care that he's gone, so why give him an explanation that will only make Guts look like a pathetic loser in his eyes?
Like to be fair, I don't think Griffith is that communicative with Guts either. He has a few vulnerable moments (eg "Do you think I'm cruel?") and there's the great staircase scene where he can't give Guts a logical reason for saving his life from Zodd, but he never tells Guts that he considers him a friend, that he needs him on a personal level, etc, and Guts overhears him saying the exact opposite. He demonstrates how he feels through his actions, but again, Guts is very ill-equipped to pick up on and internalize that.
I don't think Griffith can tell Guts how he really feels because he doesn't understand his feelings himself and he doesn't consciously think of Guts as a friend, but yk. That's why the miscommunication street goes two ways here.
As for reconcilliation, I don't really think it's likely, at least not in a happily ever after sense. What I can see happening is an admission that their feelings for each other still exist, at the climax of the story, followed by one or both dying/killing the other. But tbf that's also what would satisfy me most. Especially if they die together, I think that's my ideal.
Thanks for the ask! Hope this explanation makes sense.