Tarou feels very like an emotionally stunted child, just the ways he goes through his emotions and his severe dependence on Umi just feels like him having Umi as the only mental support he has,
and considering how his childhood went, him seeing his grandma die in 100 days, him telling his parents about it but they don't believe him (this could be the cause of why he insists on doing everything himself but could also not be), and Umi just forcefully pulling him out of the depressive episode he had when he locked himself away could explain his overdependence on Umi.
On another note, this situation just feels a bit too forceful on Umi's part, I get that she has good intentions, but this just feels too aggressive of an approach to handling what could be called Tarou's mental illness.