Cas' mission was to save Dean Winchester. And he did. He saved him from hell. He saved him from angels, demons, Leviathans. He saved him from his own self-hate and self-loathing. He saved him in all the ways imaginable.
"Dean Winchester is saved."
Cas experienced true happiness by 1) confessing his love to Dean, 2) making Dean realize how wonderful, caring & loving he is and 3) saving his life one last time.
Find me a better, purer and more selfless character.