The one thing I really like about the relationship between Eren and Levi is how careful he is with Eren. He doesn’t have to constantly cheer him up when he’s down. He doesn't have to treat him as an equal since he’s his superior. He also doesn’t have to accept the nature of Eren. Yet he indeed does so and even considers him a monster with fearsome beauty. He doesn’t owe him anything, but still gives him everything.
What’s also interesting about them, especially in the beginning of their relationship, is a stranger walks into this teen’s life fully aware of his essence and instantly takes him under his wing. Because he knows no one else can handle Eren or even begin to understand him for what he actually is. They would just tamper with that beautiful monster he has inside of him. And he for sure as hell won’t let that happen.