Dean sings only occasionally.
Sometimes he sings when it's just him and Sam, sitting in his baby, listening to cassettes and for a small time, forgetting what they do.
Sometimes he sings when he's in the shower, or researching. Just humming, under his breath, barely audible but it's still there. That's when Cas first notices.
Sometimes when Sammy's asleep and Dean's alone, he'll get out a picture of his mom, and sing quietly to her, and then go to bed with a bottle of whiskey and a determination to drink himself into a state where he doesn't remember how crap their lives are. That's when Cas decided Dean needed his sleep that night, and Cas could do the job without troubling him.
And sometimes, very rarely, when Cas was a fallen angel and was feeling increasingly wretched with every growing hour, Dean would put Cas to bed, and softly sing what his mother sang to him when he couldn't sleep. Something that gave him great comfort and the knowledge that above all that happens, no matter what, he was always loved.
you are my sunshine
my only sunshine
you make me happy
when skies are grey
you'll never know, dear
how much I love you
so please don't take
my sunshine away