I actually tend to stop playing when I know I am close to winning because I know how I will feel once I beat it. Such games I have stopped playing because of the fear of never playing include Oblivion, Fallout 3, Dragon Age, various Pokemon games, I also recently got very attached to Borderlands, but I have stopped in fear that I am close to an end.
More to the subject I have gotten very adicted to some games to finish that it was like a huge interest in many different parts of it, but fizzled a bit afterwards: Halo Wars, I hated it at first, but months later tried again and once getting the hang of it I did little else but play until I beat the campain. Dead Rising is another, I finaly got the courage to complete it and I was up until 5 in the morning so I could get the mega buster, I did kind of return to it a little while back, but my attempt at playing perfect did not go well, and what was a two day obsession stopped. I got quite addicted to Silent Hill Homecoming aswell, while playing I thought that I probably should go for replay, but again the motivation dissapeared.
I also tend to not only get obssesed with playing the game, but checking up several game information while playing for some help, or looking at background information, TV Tropes pages, wikipedia, youtube and pictures.
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Through dream I travel, at lantern's call
To consume the flames of a kingdom's fall