
I think, Mr. Cat is reincarnated by his past life, which is the dog, Winter. I don't know the main reason why he cannot tell Seungwoo but I think, Joonwo killed the dog because Seungwoo divert his full attention to his bwloved dog. I know it sounds weird but I think, Joonwo has some kind of illness or whatsoever. Lack of attention, maybe?
I just creeped out when Seungwoo interacts with Joonwo

Side dish romance is okay. There is nothing wrong with that. But if they go with full romance and no adventure or action. I'm gonna limit myself of reading this.

https://readlightnovels.net/only-i-level-up.html look back a few pages to find my comment on where we are in the story

I don't know if I'm going to continue reading this story. I felt like, the phasing of this is unprogressive. Too cliché plot. I don't know if this is right to say but it was like a rift-off of SL (︶︿︶)

http://www.mangago.zone/home/mangalist/848680/?filter=&page=1
Here is a list of manga with potential to be good reads.
As you see, I am skeptical on the old man and his old friends. Idunno. If I am on his position, I definitely be vigilant because you don't know what will happen if you continuously being reckless. And I really hate betrayals.