Comment by Yaoi Whispers on Oh! My Assistant - Chapter 36

So his friend had a very jaded view on life. He was never happy even before the accident. His philosophy of finding an ok wife just so he can check the boxes society/his mother chooses for him, it's a great example of this. 
I can definitely see where and how he's lost his purpose. Well I shouldn't say lost it because in truth he never had one but his feeling of loss would be true. He's never had to think about what he likes and wants because doing those things was never an option. All of a sudden they can be, only, he has nothing he wants. 
He was always at muyeongs beck and call so I can also understand how he could be unable to tell that he's crossing the line.
However, having said that I believe he knows what he's doing and despite having a real sense of loss and depression he is also taking advantage of the situation. 
It is quite easy to both have a condition but also exploit that condition. 
Muyeong needs to put his foot down and set boundaries but we've already got plenty of examples of he's terrible at doing that.
Seohn very much needs to stick to his terms and break up with him if he chooses his friend otherwise Muyeong will never learn.