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The first time Castiel thinks Dean is beautiful, he is piecing him back together to drag out of Hell. As he puts together his features, the pieces of his soul, it’s hard for him not to notice the inherent beauty of Dean Winchester, the love and devotion that are an essential part of him. At the time, he chalks it up to simply noticing the beauty of his Father’s creation, and keeps on with his work.
The next time Cas notices is when they are back in 1973, watching the unchangeable moments of John and Mary’s life play out before them. Cas appears to take Dean back to the present, and as he grips his shoulder, Dean turns back to him with a look he is unable to fathom. The thought goes through Cas’s mind unbidden. He’s beautiful. The context of this thought is foreign to him, so again he pushes it down, and continues with his work, continues to be a dutiful angel.
Cas first notices beauty through Dean’s actions when he refuses to allow them to destroy a town to stop a seal from breaking. He doesn’t know that the angels are told to do what he says, but his refusal is adamant nonetheless. This reaction to Dean’s refusal, his need to protect, sparks something in Cas. After Samhain is defeated, while he and Dean sit on the bench watching children play, he admits his doubts and fears out loud for the first time. He doesn’t admit that their foundation seems to stem from the beauty he noticed in a single human.
The thoughts stay with him until he is yanked back to heaven, reminded by force of his purpose on Earth. He manages to keep his mind focused, until he is alone again with Dean, tasked with keeping him from his brother and letting Sam break the final seal. Dean confronts him, telling him why humanity is worth saving, why freedom is worth having, and once again it hits him like a freight train. He’s beautiful. Moments later he is helping Dean to escape, going against heaven’s orders, and his life is forever changed.
From that moment on the thought rarely leaves his mind. The years go on, many things change between them, their lives go all manner of directions, but Cas always comes back to that one truth. He’s beautiful.
It’s not until Cas falls, becomes human, that he realizes what emotion has always been accompanied by this continual thought, what caused him to rebel, to give it all up for Dean Winchester. Love.
Believing Dean would never return such a feeling, he continues on loving Dean, continues to privately indulge in his beauty, as they fight gods and monsters for years. Through the ups and downs, the possessions and deaths and resurrections, these two things, his love and Dean’s beauty, remain constant.
When the Empty forces him into a deal to save Jack, tells him that it will come for him when he experiences a moment of true happiness, he accepts without much thought. In the back of his mind, he knows what would trigger it, knows what that moment would be, but also knows it’s never something that will come to be. So he keeps on living as best he can, with that ever present ache more prominent now than ever.
The moment when it all comes to a head, comes in one last Hail Mary effort. He and Dean are in the bunker, Death knocking down the door, moments from their defeat. He turns to look at him, and all he can think is still beautiful. Still Dean Winchester. And he knows what he has to do.