Waht 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://th04.deviantart.com/fs8/300W/i/2005/303/7/6/Kim__s_Puppy_Dog_Pout_by_StarDragon77.jpg
Update: pS. it not like a crying expression.
Update 2: I mean in terms of body language when a girl gives me this expression whats she trying to communicate?
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