Even though Castiel was made as an angel - a machine without emotions or feelings, he's raised (or fallen if you will) and discovered the power of feeling. Apart from Sammy, Dean loves him the most for he is his best friend.
He is loyal, but not stupid. He can tell when loyalty becomes an obcession. And although he did some bad things, he always did them with good intenions. You know what they say - "A path to damnation is paved with good intentions"