Dean couldn't let go.
No, he doesn't want to let go.
Letting go means that it is the end and Dean refuses to let it end this way. He knows for a fact, that there is nothing in the world, God or the Devil that can pull him from Castiel.
The bleeding won't stop.
No matter how hard he presses against the wound, the blood just flow around his hands like water. The small pool around them is steadily getting bigger.
Castiel's breathe is getting smaller and shakier which each intake. His blue eyes are darkening around the edge as if someone is slowly turning off the lights behind them. Pretty soon, he won't be able to see Dean over him anymore. He reaches his hand out to grab Dean around the neck to pull him close.
Dean can feel Castiel's bloody hand grabbing his neck, but he doesn't want to look away from Castiel's injury. Thinking that if he just stares at it hard enough, then maybe it would stop pushing out the precious blood the Castiel's body still needs.
"Dean?"
Dean had snaps up at his name; green eyes looking straight into an ocean blue which is slowly darken like a storm is coming in.
"Hey, Cas. Just hold for a moment. See the wound is not that bad. It's not that bad, we can fix this. We will get you patch up in no time. Then, when you get better, I can kick your ass for scaring me. You just need to stay with me. Okay, Cas? Don't give up on me now, buddy."
Castiel whispers Dean's name again, progressively pulling him closer, until Dean was nose to nose with him. Dean's hands never pull away from Castiel's injury. Castiel gently tilts his head back to press a light kiss to Dean's lips.
Dean was shock for a second, before he deepen the kiss. Giving it all he had, trying to express everything in this one kiss. The joy, happiness, pain, frustration, anger, forgiveness, and lastly lost.
Moving away from Castiel's lips, Dean whispers while tears track down his face, "Please, Cas. Please, you can't leave me. I have no one else. You are the only one I still have. I can't lose you too. Please, don't make me go through this again."
Castiel just gave him a small smile and with his last breathe just says, "I love you, Dean Winchester. I'm sorry."
Dean felt him go limp under his hands. Dean just stares at him.
"Cas?"
"Cas."
"CAS!"
"CASTIEL!"
Dean didn't get a response back; he grabs Castiel and shook him like he was just trying to wake him up, but still there was no movement from the fallen angel.
Dean pulls Castiel to his lap, holding his body in his arms, slowly rocking him back and forth. Dean's tears falling and hitting Castiel's shirt.
Dean was holding his best friend.
"He's tough for a nerdy little dude with wings."
Dean was holding his crush.
"Because without your power, you're basically just a baby in a trench coat."
Dean was holding his partner in crime.
"So, what, I'm Thelma and you're Louise, and we're just gonna hold hands and sail off this cliff together?"
Dean was holding the love of his life.
"We're family. We need you; I need you."
Dean was holding the last thing he had left, that kept him tether to this world.
It wasn't until sometime later, after Dean gives Castiel the Hunter Burial. After putting his handmade gravestone at Castiel's grave site, right next to another handmade gravestone with the name 'Sam Winchester' carve into the wood.
Only after everything, did Dean realize that that was his first and last kiss from Castiel.