Dean's love for Cas was so deep and grief so strong that it made Jack bring Cas back from the Empty. The kid was really like: I gotta bring my dad back so that my other dad can function normally again 😭
god in hindsight dean was so funny. he is incapable of conceptualizing relationships outside of a family dynamic so any time he got even a little friendly with someone over an extended period he'd just slot them into Family. I like you so you must be dad, mom, brother, sister