If you are not sure, you haven't! And it doesn't have to be something that necessarily made sense in a logical way.
Mine was a few years ago when I had the opportunity to hold and examine an original of a rare print book which I had been researching. The effect was astonishing, beautiful sunsets, glorious works of art, no comparison. I still have no idea why I found it so amazing, in retrospect it seems quite out of proportion but it is something that really made an impression on me.
yes I know, book...impression...sorry!