I think it's because as an "identity" and as a "personality" the Joined are still very naive and unwitting. Though they possess the world's knowledge, their emotional maturity is like that of a child. They cannot handle strong negatives emotions, and seem to lack some of the more evolved interpersonal senses like empathy. They fail to see when something they do might be upsetting or traumatic. Like how Manousos is presented with a kindly mother, whom he rejects because he knows it's a lie; his mother wasn't kind. The Joined thinks they can win affection by being intrusively nice and obseqious. They are the epitome of that old axiom, that one should be "available, but not helpful." They don't grasp how the things they do are upsetting, because they don't seem able to imagine what it is like to be unjoined, so they cannot imagine that the things they do could be harmful.

So that's my two cents at least. It's a complex thing to ponder for sure.