What does it mean when your boss pats you on the back?

He seems to spend a great deal of time with me and likes to discuss things and he always smiles and such. I don't understand why he is so nice! maybe I am looking into this too much but today he patted me on the back. I didn't know why. It was pretty random. Maybe he was thinking... This girl deserves a "pat on her back"... I have no clue! Any ideas?
What does it mean when your boss pats you on the back?
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