I don’t understand, why does Dojin go back to that dream? Because he dies or because Yel...

The nightmares are due to the poison the yellow dragon have to release when they have sex

Yellow dragon can’t die. Dojin enters the dream when the yellow dragon is forced to sleep, or in a weakened state. I think it’s showing how they are connected as a single being, so Dojin is experiencing Yellow Dragon’s dream, but Yellow Dragon is a god so his dreams bleed into reality. I think it’s playing with multiple timelines and alternate realities. If Dojin ever truly returns to his world/his reality I think there will be some significant differences from when he left.

Because everytime he'a about to die, yellow dragon will transport him to his dream until both of then wakes up from it. This is what happened last time when he was poisoned too. In latest chapt he is about to fall off a cliff. If you read the raws there is a chapt yellow dragon is explaining dojin has died a few times in Yooguk actually and he wasn't able or probably was too late to do anything when it happened.

Thank you so much, now I understand better

Thank you! Makes sense now