Due to the resets he did as Flowey, people became predictable to him. The same lines of dialogue, over and over. This is all they are and ever will be. Children are selfish by nature, but add a god complex as the cherry on top and the powers to feed it? He assumed he knew everything about everyone, that he knew them better than they knew themselves. But people are more complex than that. Part of growing up is learning to see outside yourself, even if their experience doesn't match yours.

Would he really be happy if someone decided his happiness for him? He barely knows Kotetsu. What right did he have to think being with Barnaby was all he'd ever need from one conversation alone?

"I won't do it again. I just wish I knew a way I could stay... without hurting people or taking their souls."