Check out some of the language in this caption to explore.
Strong exclamations expressing surprise, anger, or impatience:
For God's/Goodness/Heaven's/Pete's sake!
Oh my God/gosh/goodness/word
That's crazy! You're crazy!
Are you insane? Have you gone mad?
Idioms involving "think:"
Just think - we could be in Hawaii right now
Think, you fool!
Let me think about it...
He works at a think tank
Don't mind me, I'm just thinking out loud.
I think so, but I'm not sure.
Leave me alone and let me think!
You have to think outside the box
And lots more where that came from - just google "idioms with the word 'think.'"
More to explore: why does the author use the present progressive tense? Why "is being" rather than simply "is?"
One answer might be that the present progressive emphasizes the continuing nature of the cat acting nice...a process that might have a dreadful end! If the caption read "Why is he so nice to you" we would not have as clear a sense of the continuing and confusing nature of the cat's actions. Obviously the cat is not simply being nice once....he is wooing the mouse into a sense of false security.