I must admit that almost half of my game's catalog is unfinished, for several reasons. The main reason I left games unfinished is just the game makes no effort to keep me going. The reason I game most is for its stories and characters. If the story fails to entice me or the characters are unrelatable and/or unlikable, I just let it go often. Gameplay, even the best gameplay, get repetitive with certain speed, so a game with great gameplay but so-so story will be left in the backlog relatively fast.
But games who have a great story and characters, even with lackluster gameplay, can hook me to the very end. I have this desire to see the end, how everyone I met along the ride ends up, how everything finishes. I can even withstand a bad gameplay if the story is good enough. But if you combine both so-so gameplay and characters/story, I probably will not play the game more than a couple of hours before tossing it aside.
I always says that a game should not become a chore. The moment you start playing because you paid for the game or just because you don't want to left it unfinished, the purpose of a game, of entertainment, is lost and you will just force yourself for a bad time.
Different people have different reasons to play games. But all those reasons must be because you can draw a sense of pleasure out of the game. The moment this sense is lost, is the moment to put the controller down and go do something else.