"Why can't I be with him ? "

Angel and Yones story is actually pretty sad if you think about it a bit more. Hyotaru ( Angels older brother ) tells her that she's not allowed to have contact with Yone. But she still loves him even if he broke her heart first. Whenever Yone is not with her she's sad. Sure they will be together one day. But Angel is getting older while Yone stays " young ". She'll die and he'll be alone again. I don't think he would give up his demonlife for her.