I think this starts off with voracious reading of other subjects that may have interested you as a kid, but that you never had the time or inclination to learn.

In order to stave off the slightly depressing feeling that your original childhood passions are waning, you pick up new subjects or skills to learn so that you can re-kindle excitement. This is certainly what has worked for me. It helps that you're probably naturally curious about almost anything to begin with (apologies for the gross generalization).