I think this is probably the most powerful Dean-Cas scene in the entire series. At this point, Castiel has lost everything: His true family, his powers, his true form … everything he once had, all the traits that characterized who he was are gone. But in this scene, you can see that the loss that pains him most is “his” Dean. Maybe with Dean, he could have made it, but Dean too cracked when Sam said yes, and Castiel buried himself in drugs, women and sarcasm, hiding behind the mask of some strange kind of hippie guru.
But then, Dean - 2014!Dean who stopped caring about anyone long ago - asks him to be part of a suicide mission, and Cas agrees. But when he says “of course”, he’s looking at 2009!Dean, and it’s like he’s saying: “Look, whatever you become - whatever I become - one thing will never change. I will always love you and if you ask me to die for you, I will.”
In a way, this is one of the few prerequisites about Cas the writers never touched: Even in season 7, it’s all about Dean for him. What he did, he did because of Dean, and when he was about to die, his only concern was Dean. Dean, Dean, always Dean.
And though Dean will never love Cas the way Cas loves him, this doesn’t change a thing about his feelings. No matter how badly Dean treats him, how much he defies their friendship, Castiel keeps on loving him as if it’s the sole purpose of his entire existence.
So Cas is definitely right: It’s what you might call a tragedy from the human perspective …