I don’t understand, why the rest of human (doors) get miserable when they become doors and end up dying except

Yaoi logic if he dies our top wouldn't have anyone to fall in love with and have hot ass sex with

There are a number of reasons for that. Mainly the fact that (unlike Cain and Gyeong Joon) other demons use humans purely as doorways and don't actually form a relationship with them. The humans are basically in a constant state of terror and being a "door" interferes with their normal lives and relationships.

And aside from Cain being the way he is, Gyeong Joon had been living with the pain and loneliness since losing his fiancé. So he had a void that Cain was immediately able to fill, and gave him the opportunity to heal and fall in love again.

I remember them saying that MC's soul was strong ╮(￣▽￣)╭

That will be answered in future chapters. Gyeong Joon is special (and sexy af)

I'm totally not ready to re-read that section again in English because ( T﹏T )

I just can't wait for the translation to get to chapter 50-55 <3333
....but since I've already read it, I know to prepare a bucket, mop and box of tissues for my tears and....yea xD