Season 7. The two learned more about each other and started to grown on one another. Eventually 😄 the two comforted each other and slept together, not realizing in a few short months, Meg would be carrying a 😄 Nephalem, their unborn child, Cassandra Master.

But then, in 4.04 Metamorphosis , Dean says to Sam: Cas 😄 said that if I don't stop you, he will.  That was the first time he ever calls Castiel 'Cas'. 😄 And I wonder if it's because he got a bit comfortable with Cas after 4.03 In The Beginning , 😄 when Cas laid a hand on his shoulder and gave him a sympathetic look.