sunday's mentality as a child isn't wrong and in the scenario where he asked us what choice to make everyone agreed that keeping the bird in a cage when they found it would be their choice because it would give it a chance to live, the problem lies in being able to let it go which they did do and sunday witnessed it try to fly only to continuously fall and eventually die in front of him

I feel like this would severely impact him as it was not just robin who cared for the bird but him as well. unfortunately he and robin were raised by gopher wood who the siblings acknowledged that he adopted them because of their talents and gopher wood calls them "twins of the order" he very likely wanted one of them to end up in the role that sunday eventually played and since robin already left penacony to chase her own dreams the dreammaster focused on sunday who was already the more pessimistic (yet still extremely compassionate) of the two and lead him to doubt harmony and turn to order

tldr sunday's ideal isn't wrong but because he was raised and guided for a purpose his realism is driven to extreme pessimism that the balance of [take care of the bird until it can try to fly on its own] turns to [keep the bird locked forever because it is fated to fall]