
I think it's cause his attitude is mainly cause of his past. He was abandoned and so a way to cope with that was to look for interesting things, including killing himself and him dying. However with sayo his childhood dreams can be completed through him. Sayo is someone he needs cause he is teaching him these things he never had. Opposites attract I think
Sayo grew up in a loving home while Yama the exact opposite. Yama grew this personality on account for that and sayo is teaching him (and the people who he interacts with) just by being around him. His loving attitude and innocents changes these people.
And when he let go of Yama it was a natural reaction cause he freezes when he's next to women. Yama is changing in that he doesn't want to die (that's how I saw it) and it's an exact reflection of how he was in the train scene. He was expecting sayo to save him and has even depened on him. Yet he didn't so now he kinda closed himself off again
He's a very unstable person cause of his trauma. He has been through a lot and is only learning at his age. years of habits build up it's not going to be easy to chang. He is slowly changing and so is sayo
That's my opinion anyhow

Very fair thank you for responding, I understand all of that to an extent and I agree that yamadera is changing a bit and in that moment he realized he didn't want to die yet and now that you mention it I see the abandonment parallels but he still annoys me a bit lol. Also, the reason Sayo let go isn't because he's like that around women, it's because of Mei's power as a vampire immobilizing him and kind of making his body go slack

I thought it was both.
At first it was shock but the bag knocked him out of it. Then it was her and he had to stab her and by then bros been saved. Cause he blushed and his eyes and the second one he had the black thingies around him
Thats fair tbh. He has a reason for acting the way he does but the way he acts sometimes does kinda bother me too ://
Yamadera is pissing me off man I don't get why he acts the way he does sometimes. Specifically in regards to Sayo. Like dude why are you all of a sudden mad that he didn't save you I thought you wanted to die at least a little..? And isn't it obvious that he didn't willingly let go?? Fuck is your issueeee