Because you like Danny Donahue in Role Models
Castiel might be an immortal angel, but the human world is still more than a little confusing. See them in Supernatural.
It's hard to tell if Dean Winchester cares more about saving the world or his 1967 Chevy Impala. See them in Supernatural.
A wildly brilliant detective, Sherlock Holmes can figure out everything except how to interact with other human beings. See them in Sherlock.
After surviving consecutive fights-to-the-death (damn you, President Snow!), Katniss Everdeen is thrust to the front lines of a real war. See them in The Hunger Games .
He's an eccentric pirate making it up as he goes along. He's Jack Sparrow, savvy? See them in Pirates of the Caribbean: the Curse of the Black Pearl.