Castiel was a quiet man, one who never gave any indication about what he was thinking or feeling. Everyone thought that he was heartless with no feelings inside him at all. But, Dean saw his emotions when no one else did. Dean saw the emotions that swirled within him because he looked into Castiel's eyes. It was in his eyes that Dean saw all that Castiel tried to keep hidden from the rest of the world.
Contrary to popular belief, Dean had to work so he could read Castiel. At first Dean tried to read Castiel's body language, not long after he gave up on that approach because Castiel's body was always engulfed inside an old, tan trench coat that made it impossible to read him that way. Next Dean tried to watch Castiel's hands but found he couldn't because his hands were always steady, even when looking Death in the eye. After that he tried looking at the other man's jaw. This approach was squashed once he found that Castiel always kept his jaw locked in place, never even letting it fall open to let his surprise known. After watching people all his life Dean had found that their lips would occasionally give way to their feelings. So naturally Dean tried this on Castiel, and failed because the man never un-pursed his seemingly permanently chapped lips, as though he was restraining his thoughts from falling from them and playing with others. The next trial and error was the study on Castiel's eyebrows which always seemed drawn together, as if always puzzling over a difficult problem in math class. Finally Dean looked upon Castiel's eyes, a deep blue, one that was darker than the ocean but lighter than the stormy sky.
It was in Castiel's eyes that Dean found the emotions that Castiel tried to hide from the rest of the world.
After looking upon Castiel's eyes Dean began to notice the way his eyes went lighter when Dean called him Cas. He began to see the way his eyes would become dark as a storm when someone threatened Deans safety. He saw the way his eyes seemed lighter when swimming with tears as another of his brothers or sisters tell him that he ruined heaven. Dean even began to notice when he tells Castiel that he is his family, that he needs him, how his eyes stay light and full.
When people ask Dean how he is able to read Castiel's emotions, he just smirks and tells them to look where the ocean meets the sky. They always give him a confused look as they turn away from him, but Dean finds that it delights him to know he is the only one to find the emotion in the blue.