If one of your players feels the need to drop out of your campaign, what should you do?

This hot button is relevant for me because I’m actually in this situation. I’m currently running a campaign that the rest of the group is thoroughly enjoying, but one player simply isn’t into it. It seems my choice is whether to end the campaign or go on without him.

When starting a new campaign, this isn’t an issue. I never start a game without buy-in from everyone in the group. I’d also never say to my group “I’m going to run this Trail of Cthulhu adventure, but it only works with three players, so two of you have to sit this one out.” It’s simply common courtesy to play with the hand you’re dealt.

But what about the opposite end? What if you had buy-in from a player but it just simply isn’t working for him? Should he have to drop out, or should it be common courtesy that the “buy-in” should be on a session-to-session basis? Alternatively, should you split the difference and wind the campaign down quickly even if you have several months of material left?

Before releasing the question I do want to point out that, in this case, the player enjoys the adventures and the roleplaying; he just can’t stand the system. Furthermore, he’s more than willing to sit out until the next campaign rolls around. Yet in spite of how easy he’s making it on me, as a GM I still feel a certain responsibility to keep him in the chair. It doesn’t feel any different to me than if I’d banned him from the beginning.

So what say you? If you are running a campaign that the whole group is enjoying sans one, would you end the campaign or let the player sit out? Are there any specific circumstances that would sway you one way or the other? Is it case-by-case or do you generally fall on one side?