It happens. I'm currently trying to decide if I'm going through burnout, am too busy for the commitment I'm used to putting into the game, or if I just need a change of scenery.
I finally killed my (seemingly interminable and painful) 3.5 campaign just before Christmas. After a month off, we're doing our first game of 4e tonight and I'm kinda psyched just to be doing something different. We're only doing one module, then moving to nWoD (Hunter), but the vote was close enough, and I wanted to be able to formulate my own opinion of 4e before leaving it behind for a while.
If 4e tickles me, but nWoD doesn't, I'm going to chalk it up to time commitment. If it's the other way, I'm going with change of scenery. If neither is fun, I'm burned out.
When all is said and done, I suspect there are multiple factors. We've got four kids and are gearing up to build a house (which I'll general and do some work on myself), so time is a huge issue. On the other hand, I did a lot of system switching in my formative years, so sticking with any system for more than 18 months (a semester in school was about right) gets painful -- meanwhile, I know at least one person (and probably the majority) in my group would rather find one system to use henceforth.
I suspect the next twelve months will be pretty interesting for my gaming. I see a non-trivial possibility that I may need to find a new group or stop gaming. Since I view gaming as a social, friend activity, the latter is more likely.