Dean was way too young to remember their first meeting, but Cas - who was almost ten back then - remembers it perfectly. It was the first time when he saw a baby that was not a wolf- such tiny, warm, sweet smelling creature that smiled at everything, including Cas. Castiel’s father put his hand on the shoulder and told him that leading a pack means not only taking care of the wolves living in it. And then Mrs. Winchester let him hold little Dean for a while, and Cas was fascinated with his curiosity and complete lack of fear.
He never thought he will meet Dean almost 22 years later, though.