What does the "PUPPY DOG" expression mean?? PLZ HELP!!?

I get it from girls all the time. I dont know what it means. This is the best picture I could fin online.

http://stardragon77.deviantart.com/art/Kim-s-Puppy-Dog-Pout-24634190
pS. it not like a crying expression.
Update: I mean in terms of body language when a girl gives me this expression whats she trying to communicate?
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