A few possible explanations:
• Rick Perry is just a down-smiler. (This is similar, but not identical, to this condition.)
• Rick Perry has just remembered that all mortal men must die.
• Rick Perry is trying to do a difficult logic problem in his head.
• Rick Perry wants to look stern and disapproving in the back of all pictures with President Obama, and this is the closest face he has.
• Rick Perry has just forgotten a word that he wanted to use and is trying to summon it to mind by going through the letters of the alphabet, starting with A.
• Rick Perry’s glasses did not make quite the impression on President Obama that Rick Perry hoped.