Didn't all his colleagues tell him their dying will to keep yoonseo alive?

Yes that’s the second reason. They all basically knew that even if they survived, they’d probably end it due to the PTSD, survival’s guilt, etc, so they decided to give the youngest a reason to live even after the dungeon was over T-T

And they knew yoonseo would faithfully follow their wishes this is seriously ruining my mental health

You and me both bestie T-T