If there is a fun thing to do that is inefficient and a horribly boring thing like smacking yourself in the face with a shovel next to it that gives more XP, players will do more XP. They'll try the fun thing once or twice but then go, 'No, I can't help it. I need to hit level 50. I want my end goal more than I want my journey.' So it's very easy to have the journey trivialized.
Achievement is the love of watching bars grow -- that's our industry. I don't think there's a more fundamental human need that gets tapped into by these games than watching your bars advance, and that feeling of progression -- of being able to say, 'I am tougher than I was before.'