Satoru wasn't scared of anything. When he went to go fight Sukuna, he knew it would be one of the biggest fights in his life, knowing that he'd probably end his life by going there. But he went anyway.

You had died to Sukuna. He brutally murdered you, burning half of your flesh and blowing your head off, then cutting you in half?

Satoru hated him forever because of that. He went to fight him to avenge you, but ended up dying himself with a smile on his face.

And now, here he was... In the afterlife with you, sitting on a couch so casually. "How'd I die?" he asked, looking at you. "I was fighting Sukuna and he had me at a weak point, then chopped me in half. No biggie."

No biggie, he says. He's obviously accepted being here, huh.