Betrayal. It cut him so deep that all the guilt, grief, and anger that had built up inside of him since he met him rushed out the moment his angel walked into that lake. It was a never-ending, sticky, and thick flow that gushed out of him, making him choke on self-loathing everyday.
Dean tried to find solace and peace in fixing a few cars at a local garage in town and tinkering with his '67 Chevy Impala he was so fond and proud of. He tried to ignore the image of tortured cobalt- blue eyes, black, endearingly messy and dirty hair, and a blood-stained and unshaven face that stays constant in his mind. He tried to forget about the bloody, un-washed tan trench coat still wrapped up in the trunk of the Impala, serving as a constant reminder to the hunter of an innocent being lead astray. He couldn't bring himself to throw the coat away, yet he can't bear to look at it. He tried to lose himself in the work, but most of the time, he catches himself thinking about everything: his life, the times he screwed up, and how life was before the angels stepped in. Almost immediately, he would remind himself that if the angels didn't barge so rudely in his life, he would never have met the greatest yet most tragic love of his life.
During the middle of the night, when he thought that Sam and Bobby were asleep, Dean would sneak down to the kitchen, grab a bottle of Jack Daniel's, and just drink until those blue eyes faded away and he fell into unconsciousness on the hood if the Impala. The first time Sam saw his brother passed out on the Impala, he merely smiled sadly and left Dean to sleep under the stars.
Every night that Dean slept on the hood of the Impala, the hunter would find himself dreaming of that lake where Castiel had visited him from long before. The first time Castiel had visited him, the skies had been light and clear. The second time, the stars shone brightly and warm, colorful nebulae swirling beautifully against the dark canvas. Castiel had his face tilted up towards the swirling painting, and Dean remembered how the angel's face glowed and sparkled, a kaleidoscope of colors playing on his face. He remembered barely stopping himself from claiming those horribly chapped yet beautiful lips, barely keeping himself from jumping the angel right there. His angel was unearthly, beautiful, and for those few precious moments, his to feast his eyes upon.
Now though, the stars and galaxies were all but a faint glimmer, monuments reminiscent of times when Dean still had his angel. They did nothing to illuminate the dead, still waters, the dried, shriveled grass, or the bare trees that now dominated the landscape. It was as if every familiarity, love, and feeling that Dean ever experienced at this lake had died alongside his angel. Even so, he keeps sleeping on the Impala; he keeps dreaming about this place. Dean remembers how sacred and beautiful everything looked when Castiel was around, and misses it so much that the hole in his chest tears further apart, sending waves of agony pulse through the human's chest.
And so every night, the hunter mourned the angel he never truly had, the stars and the galaxies draining his life away into a dark, deep void.