I've noticed this as well. My theory on the phenomenon is that putting something in your own words is a big step in understanding. If you've read so8res' recent posts, his studying method involved reading, doing the exercises, then explaining it in his own words, even if only to a text file.

I think explaining something really solidifies the knowledge in your mind and gives you a better understanding. That itself could make you more enthusiastic, as it provides validation that your hard work has paid off and that you finally understand it.

I find it similar to the way that once you tell a story, you tend to tell it the same way every subsequent time, even if it's fairly long. I have no proof, but I suspect that the act of giving an explanation (or telling a story) forms some kind of very solid and available memory.