if you don't trust people and have faith in them you can't have a serious and strong relationship, that's way Chiyo needs to change. Her mistrust was ok at first, they didn't know eachother well etc etc, but now that need to change because she can't vent on him everytime she fells like she's betrayed
This manga is opening some nice conversations (≧∀≦)
Well it's not like he wants her to be a completely different person, he just wants her to open up about her troubles so he can help her. Otherwise the relationship just won't work out.
I think that when you fell in love you never entirely know your other half. Like the more you get to know your them the more you see that there aren't perfect like you idolise them and there are things that you don't like and want to change because you love them anyway but those things that seems wrong to you troubles or disturbs your relationship.
I will never understand people saying that their SO needs to change.. you fell in love with them because they are the way they are and now you want them to change?? what the hell? Chiyo don't ever change (●'◡'●)