To further illustrate my point, imagine the difference between a real skater and someone who's playing the story mode of Tony Hawk's Underground. They both want to reach the top, and they both want to get better, but when the story is done for the gamer, he puts the controller aside to look for a new game so he can relive the experience of climbing to the top. The joy of skating has become undermined by the motivation to finish a journey, even though skating should be what it's all about.
So eventually your beloved multiplayer game has lost much of its excitement. That's where what I like to call the reset button enters the picture. An offer comes along to start anew. As long as you buy this freshly-released game, you can get to relive the excitement you felt when you first played the previous one. And not just you, but everyone else will be able to reset their game, so you won't feel like you're alone in experiencing this. It's a new adventure for everyone!
Except it isn't. It just feels like it is. To its bones, it's the exact same thing you played before. Sure, they might have added some new perks, maybe the graphics are a little cleaner, perhaps they've even changed the setting to something different. But that's not what you're looking at when you are watching the gameplay trailers, is it? No, you're watching as that insanely addictive +100 pops up after an enemy player has been shot down by whoever it is that's in control. You gleefully observe as a skillfully thrown grenade earns the player a killstreak. You foam at the mouth as a carpet-bombing is called in on a group of poor sods bunkered up in a building. It's not exciting because it's new, it's exciting because it's familiar.
Another case of the reset button would be with rhythm games like Guitar Hero and Rock Band. When a new game in one of these franchises is bought, much of the excitement lies in getting to explore the setlist. Maybe you'll find some old favorites, or some new music you've never heard before. You play and play and play until you've gotten the most out of it all. Now you are left with two choices: Either gradually build upon your library by buying familiar songs via the online store, or buy another game in the franchise.
There are three major factors that play into why you'd rather buy a new game. The first and most obvious one is the almost explosive feeling of getting a completely new setlist, which brings an immediate addition of several hours of gameplay and subsequently reintroduces the opportunity to explore unknown content. The second factor is related to what part the games play in your social life. As you and your friends play through more of the games library of songs, the pressure to get more tunes increases. It's easy to decide that the quickest and most efficient way to do this is to simply buy another installment of the series.