If you are doing something you’re interested in, your brain will function differently and more effectively at work than it would just doing something you are able to do. Your brain is basically working off electrical signals at all times, and this is measured by the “brainwaves” or what type of energy your brain is emitting.
If you are interested in something, your brain is more “excited” (beta brainwaves, 12-38 Hz) and you will be more productive. Doing what you love will actually make you better at whatever you’re doing.
Nobody can spend all of their time in that excited state, so you do need a good work/life balance and solid sleep as well.
This explains it a bit more: http://4mind4life.com/blog/2008/02/09/beta-brain-waves/