Its something he does because the pitch is his stage and he is an actor. I am 100% sure that inside he is gleefully giggling, but because he had a little thought to himself 4ish years ago "what if I don't celebrate when I score, that will be something distinctive" (I say 4ish because he celebrated the early goals for City and Inter).
To be honest its no different from Borini's hand bite, or Sturridge's arm wiggle, just an affection he has chosen to look different
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