Oh damn, you can be sweet and supportive on these things huh, Shuichi...
It doesn’t feel like he’s humouring Kaede when it’s a down-to-earth reassurance that she needed. He knows fine well he doesn’t have to change everyone, or even solve their problems. He just needs to reach them in good faith, right?
So why is there an issue with Ouma, even in the ‘best routes’?
Well, an unwitting expectation of him may have something to do with that... something that simply isn’t present with Kaede.
One thought is, Kaede’s problem is much more straightforward, so it’s easier for him to understand that Kaede needed reassurance first. After all, it’s not always possible for people with wandering or hyper-fixated minds to ‘force their thinking’, especially an Ultimate fixated on their speciality (in this case, a creative one, and an extension of her persona as a whole). So telling them to change that would be no bueno, like rubbing salt in an open wound. This is clear.
Kaede’s struggle was with not focusing, and ‘being rude’. Ouma’s was that he wanted to reach Shuichi, but for some reason couldn’t stop lying to do so. Shuichi could understand the former as fairly common sense, but the latter not so much. The unspoken question for Ouma’s situation is ‘well why lie, then’, but in some responses Shuichi nudges him into not doing so in order to reach him first, and even in the ‘best one’ suggests he can ‘tolerate’ it.
It pretty much negates the struggle Ouma’s facing to begin with. Considering the ‘can’t love, but may tolerate’ angle has overlaps to the ‘can’t condition yourself to enjoy’ angle, it’s no wonder the reactions there are similarly awful. Though Shuichi’s secondary reaction is distraught, meaning he doesn’t exactly intend to say anything cruel or consciously reject him.
A problem may be that Shuichi is breaking up the process of his own reactions, but this only makes it harder for Ouma, because one part of that process expects Ouma to bring himself forward. The fact that he’s having a Harmonious Heart about this means it’s a serious issue for him though, and that issue needs acknowledgement first, before any progress can actually be made.
That can’t happen if Shuichi - however internally - regards Ouma’s lying as ‘foolish’ or otherwise, something so simple to ‘overcome’ that he can ‘just do it’.
Moreover, while telling Ouma that he ‘can’t imagine an Ouma who isn’t a liar’ for example, may have come from a good place of reassurance... in reality, he only reinforces what he already knows about Ouma- that... he’s a liar. This doesn’t actually address his problem in the least, or even bring anything new to the table for Ouma. It comes across as an unconscious, apathetic type of rejection, that is a willingness to only accept who he is on the surface.
What doesn’t help, is the childish and cheerful, decidedly-not-serious way Ouma takes the response from the get-go... which may be a coping mechanism for the rejection. Compared to Kaede, who ‘fights back’ and throws more of her heart out into the open, this doesn’t give Shuichi a clear ground to work with. Because of that, Shuichi sees something like childish pouting and crocodile tears, and ‘plays along with the game’ instead of really checking in on that.
After all, ‘playing Ouma’s games’... makes him happy, right? Isn’t that how it was in his Freetime Event...?
But in a situation like this, ‘playing along’ only reinforces the wall even more. Ouma’s not even trying to be cryptic, he’s only asking to be met halfway. So treating Ouma’s lies as a silly game - when it’s a serious issue, and a way of life - is not good. For perspective, it’s like insinuating to Kaede that her fixation is freakish, a burden... and to pour salt in the wound, patronise her for that.
And on some level, Shuichi does indeed judge Ouma for his ‘nebulous lifestyle of deceit’, and will even end the mode telling him it’s ‘not something to be proud of’. Even though he doesn’t actually know anything about Ouma, his life, and his situation... he still patronises him by posing as ‘more moral’ over him. As if he’s the ‘grown-up one’, and Ouma’s the ‘child who needs to be taught manners’. Regardless of what his actions really meant, even at that moment (after all, Ouma uses a more serious sprite when he cries about Shuichi having forgotten a promise... make of that what you will).
What’s more, Ouma will even break the illusion of the ‘crocodile tears’ in the Harmonious Heart event, to tell Shuichi that he’s ‘close-minded’ and ‘a monster’. So it’s made very clear that Shuichi’s suggestion of Ouma faking his tears, thus ‘playing along with the game’, was... not taken well, despite his intent. But otherwise? When Shuichi suggests he’s lying about his reaction, and also that he ‘can’t imagine him not being a liar’?
Ouma ends the event looking happy, while in one case, even declaring he would ‘give up lying’. Just like that. It’s soooo easy, right?!
Kaede expressing happiness with Shuichi’s approach meant that he felt right. And he may have been right in regards to her, because Kaede is much more open than Ouma is as a person. She is easier to read... ergo, easier to react to. But as Shuichi is, he’s ill-equipped to truly deal with Ouma’s mechanisms, and what’s more, his own biases twisted against understanding them.
So even though Ouma’s smiling, how can Shuichi really be sure he’s happy...?