Since Jack's entire world is Room, he and Ma have a lot of trouble seeing eye to eye when it comes to Outside and what is real and what isn't. In particular, since Room is Jack's whole world, he is extremely attached to it and takes it very personally when he realizes that Ma does not see it the way he does. For Ma, everything in Room is a reminder of her horrible situation that she doesn't have control over, but Jack is the one aspect of Room that isn't tainted and that Old Nick doesn't have control over. However, Jack cannot make that distinction between himself and the rest of Room; he cannot see himself as separated from it. Room is all he knows and the only thing that he knows he is a part of. Therefore, he gets very upset when Ma calls Room "stinky" and when Ma is so eager to get Outside and leave Room behind:
"'Why don't you like being in Room with me?'
(...) 'I always like being with you.'
'But you said it was tiny and stinky.'
'Oh, Jack.' She says nothing for a minute. 'Yeah, I'd rather be outside, but with you'
'I like it here, with you.'" (85)
Jack can't understand how he is separate from Room, as he is from Ma's perspective. This is further evidenced by the exchange between Ma and Jack at the end of the chapter "Dying," as Jack wants to go to Bed in Room, but Ma says they won't be going back and Jack starts bawling. Jack is deeply upset by the idea of sleeping anywhere other than Bed, as he knows nothing other than Bed. Though most children do tend to get attached to material things, they live in a world where they can establish strong connections and roots with other people. All that Jack has connections with are the things in Room, and therefore taking those things away from him is almost like taking a friend away. While we and Ma can't really see what makes the trash can in Room so special, Jack cannot imagine how anything else could live up to Trash. All he has left is Ma, and he is probably already feeling distant from her, as she seems eager to get away from everything he knows, so his closest relationship even seems to be strained.