This is very realistic. The dependence they have for eachother is very, very rare. Usually, someone like Saitou would be very tiring for their partner. Being worried all the time, worrying about wether your partner is going to kill themself is exhausting. Not everyone is like him. Not everyone has the brains to look up their partners symptoms, much less try to help. It's a really nice story, though it hits hard.
"boys will be boys".. let me go puke(⊙…⊙ )