why zuma is so addicting?

zuma is a simple game. you just hit the balls, click and click. that's basically it. it got a simple rule just don't let the ball entered unto the mouth of zuma. but why i feel so addicted about this? cos when you finished one level, you will get a sense of fulfillment without a satisfaction.that's basically it! then you want to finish the game right there and then .. kahibang. Good thing tapos ko na.