You're thinking, "what is it that I like about that image and how can I use it in my own work?"
There is an exciting shift that happens in your head and it's hard to explain. When you really grasp the principles of a successful photo, you start to look at every image critically. I don't mean that you're criticizing the image. You're no longer just seeing a subject and composition in a photo. You're analyzing why you're either drawn to the image or why the image doesn't have any impact on you. You're thinking, "what is it that I like about that image and how can I use it in my own work?"