I'm excluding addiction answers such as ones gained from gambling and idle games.
The best answer is the game fits me well, that is I love the controls and mechanics and I love the challenges given by the game. These are enough to make come back to replay the game some day.
The decent answer is the game has unlockables and or unfinished story/mystery. I'd keep playing the game but if it's too hard to achieve or all has been achieved, I probably not going to replay the game.