Okay can I say,
Number one, the way Dean is treating Jack kills me. I just want Dean to adopt the little bean.
The more I think about it, the more it kind of makes sense. Over the years Most of the people Sam and Dean have taken in, and cared about have died. Charlie, and Kevin, now They lost Cas, Dean’s best friend, and his mom. Even Crowley and Rowena (he may not have been super attached to them, but he still knew them).
Everyone Dean becomes attached to either dies or leaves him. And he is still dealing with their deaths, the most recent ones. And that’s hard, and like any true Winchester, he can’t express grief in a healthy way.
Also Jack is the son of the archangel who killed Cas, and to his knowledge Mary. So maybe seeing Jack reminds Dean of what Lucifer did? He just sees the murderer of his family.
Maybe he doesn’t want to get attached because he doesn’t want to feel the pain of losing someone else he cares about. So he feels that this way, if Jack does turn evil, or is killed, it won’t hurt as much.
I could just be overthinking it, but these are just some thoughts.