I think it has to do with the level of mastery. At some point, when you become really good at something (almost anything), you realise that a lot of your own knowledge now comes from explaining the basics to people who don't know anything. By forcing yourself to redo the basics, you get fresh insight on a level that someone new to whatever activity simply can't appreciate because they don't have X years experience with which to consider things.

I know that I now love teaching bash shell and C. I know that I am not the most talented shell guru or C programmer, but a lot of my knowledge now comes from answering questions from beginners that I hadn't previously considered (in bash, e.g., how can I tell within a script if the standard input comes from a keyboard or a file, or in C why don't you require the argument to return to be in brackets?) Questions that aren't necessarily hard, but that I hadn't asked when I was learning and now need to figure out the answer to on-the-fly. I now love teaching beginners because there's going to be the chance that they ask a really interesting question and I'm going to learn something myself.

10 years ago I would have hated that, because I wasn't comfortable in admitting what I didn't know - partly because I was still finding out that I didn't know things on a daily basis. Learning has now become a delight, rather than something to be achieved to get to the next thing.