Yuu’s mental health is literally deteriorating. He’s always had a fake smile but you can see how he’s going insane—at this point he believes ANY way of “saving” Mika is a good way even when we, as the audience, know that Mika becoming gear isn’t a good thing. The reason he refuses to acknowledge the severity of the situation is because he’s disillusioned himself so much to the point that his thought process is “everything will be fine. Mika will be fine because he HAS to be, there can’t be a world without Mika ik it.” I think kagami is finally leading Yuu into character development.
In addition to this, people seem to forget Yuu isn’t human. Who knows how his mind really works. Past Yuu was creepy as hell, the other Yuus seemed to have weird ways of thinking too. There’s a child-like simplicity to his mind at any age, almost like he’s a machine or alien trying to learn relationships and stuff. So, yeah, Yuu is kind of an idiot, but there’s an actual plot reason for that.
The fact he was more hostile and smarter before, aka the beginning and then when he got new "family" and also found Mika again his character started deteriorating and he started being more of a child I think. Now that he lost Mika again, I think his mental shock is so big that he is actually going crazy and wants to save Mika in any way (so he is willing to lie to himself that any way is good and everything will be good if he is happy).
yuu has one of the shittiest personality I have ever seen, mika sacrificed himself for this idiot unbelievable!!!