Many hunters don't begin fighting until after they've had a chance to live. After they've experienced anger, sadness, and happiness. After they've felt love, swaying awkwardly at the prom beside a girl in a slightly too-small dress who's holding her glittering shoes in her left hand because her feet hurt too much to wear them. After they've learned how it was to feel that love, express it in words and action, how to share it with another. After they've had a family, their wife or husband, a few kids, a dog, and a house with an unkempt lawn. After they've had it ripped away from them in one night, just one bad night when their spouse was home late and then was never home at all.
Dean Winchester never had a life outside of hunting. He was never allowed it. And while he did learn how it felt to experience love, he never had the chance to learn to show it, to share it. He was a handsome, rugged boy in a leather jacket who moved in from out of town with a shady past and a cocky smirk; the girls that flocked around him weren't looking for feelings. They wouldn't accept them. He felt his love quietly, smiling through the pain of silence in his own suppressed, private way.
A few years after they met, once Castiel began sincerely learning Dean, how he acted and spoke, he started seeing all the little things Dean did to show his love. All the little texts of "Be careful", "Made it to the motel", and "You safe?". Every time Cas got to sit in the front of the Impala; especially the precious few times Dean let him drive it. Even a beer, slid down the table at just the right speed, a burger dropped in front of him without comment, a bracing hand on his shoulder for just a few moments. He never said the words, but Castiel never felt that he needed to.
The first time Castiel drew his courage and said the words, Dean responded by reaching out and pulling him in for a warm hug, a soft kiss pressed to his temple, a muttered "same" against his hair. Since then, Castiel said the words constantly. He was used to white rooms and unfeeling soldiers. He didn't know how to express love, not really, not like Dean could. So he settled for telling Dean every time he realized, once again, that he loved him. When he saw a black car on the street that shined bright from a new wash, when he saw a plaid shirt on a woman walking by, when he saw the golden sun peeking through the spring-green leaves just right. After something reminded him of Dean and he felt overwhelmed by his love. He pulled out his phone and sent a text, out of the blue and expecting no response, sometimes all three words, sometimes just the most important one, sometimes just a heart or a smiley face.
Dean's responses were always swift, usually an insult or brush-off, but even in those Castiel could feel Dean's small, flustered smile as clearly as if they were face-to-face. "Dork" and "Nerd" became terms of affection over text, and then in person, as well, said fondly through a laugh while responding to a request for kiss or a hand to hold (requests that were never denied).
Dean, although still hunting, enduring what many would consider worse than death, will never be able to give Castiel enough for letting him live a life. A life with too-long stares and a warmth in his chest. Glimpses of tan cloth out of the corner of his eye that make him smile and a back against his own in the middle of a brawl. The chance to take a few moments, just a few, when lying in bed, facing blue eyes and chapped lips, to reach out and touch. A life that finally allows him, faced with the undeniable fact that Castiel is here, here for good, here to stay; the chance to build up his courage and say those three words.
And he'll try his entire life to love Castiel as much as he deserves for just responding with a small, confused smile, as if he already knew, and an affectionate, "I love you, too, you dork."