"Pigeons experimented on in a scientific study were more responsive to intermittent reinforcements, than positive reinforcements. In other words, pigeons were more prone to act when they only sometimes could get what they wanted." (Wikipedia) ...but you're much more sophisticated, you've got a much more advanced brain, capable of reasoning – you don't just reflexively press the bar for the occasional pellet.
Really? How many times have you checked your e-mail/Facebook/Twitter/texts today?