I really like this one line from a posting today on the Harvard Business Review blog:
Our past research found that, of all the events that can keep people happily engaged on the job, the single most important is making progress in meaningful work.
I think that really nails it. When I get busy with tangential work, I start to get nervous that I’m not really moving the business forward. But when I get solid progress on solving core problems, I feel really good.
Full article here: http://blogs.hbr.org/cs/2013/07/working_from_home_a_work_in_pr.html