The complete absorption in work is a symptom of a mental state called Flow [1] and coined/popularized by Mihaly Csikszentmihalyi. It's the best explanation I have for why programming is so enjoyable.

[1] http://en.wikipedia.org/wiki/Flow_(psychology)

As to the actual topic of the post, life is boring when you're not engaged. I've taken to carrying a small bound notebook in my pocket so I have something to during the downtimes.

Carrying a book with you is a very good habit to embrace. However, cultivating that habit is a little harder to come by.