Itself is a pronoun we use when talking about something that is both the doer and the receiver of an action, or when we want to emphasize that specific thing. When a cat cleans itself, the cat is doing the cleaning and receiving the cleaning. When a door closes itself, no person closed it: the door did the closing on its own.

We often use itself for emphasis, to draw attention to the exact thing we're discussing. If you say “The solution itself was simple,” you're pointing out that even though everything around the solution might have been complicated, the actual solution was straightforward. When someone says “That's kindness itself,” they mean it's a perfect example of kindness, the very essence of it.

Itself can also mean “alone” or “by its own power.” A fire that goes out by itself needs no one to extinguish it. A problem that solves itself gets better without anyone working on it. When we say a building in itself isn't dangerous, we mean the building alone, separate from other factors, poses no threat.