I aint writing the whole thing out either
though to save some people time, there is zero added understanding about Eita's state of mind in there
I can understand. Though this would be my interpretation, but his state of mind was explained from the start.
We're introduced from the couple's backstory that they're both the type to go with the flow, but that's not the actual case with Eita. One time in a state of vulnerability, he becomes very emotional at the fact that they lost the choir competition because everyone else didn't take things seriously.
He views this frustration as foolish because everyone else isn't frustrated as he is, so "why so serious, bro?" -Is the feeling he's conflicted with. He says sorry because he viewed it as childish.
This however, was the one and only time he's actually displayed his true feelings on the matter, as they continued to live a very carefree life. ..the second time would be from his death, because Sana didn't realize how much the choir song really meant to him, and his frustrations with it spilled over into emotional instability, leading to a tragic accident.
I can sympathize to him to an extent. He's an actually serious guy, but found himself to be too swayed by the flow, leading him to a life he found unfulfilling by his true nature, a desire for achievements. They never went to college, only lived a simple life, spending their free time with each other. Somewhere along the line, he acts on his true feelings of frustration and finally does get a full time job, but it's with a sketchy company because that's all he could apply to with his credentials. "You're a young man approaching your 30s, what have you been doing with your youth?" I imagined the choir song was a trigger for all that frustration to pour over. Poor Sana didn't realize all this, because she's the type to truly go with the flow, even going so far as to try and bring back Eita with necromancy. Perhaps she also realized how Eita felt in the end? Which is why she went through to this extent, to give Eita another chance in life to truly live out his ambitions.
The stinger is, if she actually was successful.
But anyway, just like Kurihara, those are just my deductions! I love these series to pieces. Mysteries like these are so compelling. Thanks to the translations!