Every year I tell myself I will keep my current model until it stops working, but every time a new model is released I buy it on the first day. I'd like to say I would keep it until it dies, but I'll probably buy the next release.
Anyways, the main reason I've always bought a new machine every year is the lack of performance in some way or another and I think for the first time this MacBook Pro is powerful enough for all my needs. Even the 2010 model wasn't good enough, so I have the 2011 model.
I had the same issue with my desktops; I kept upgrading to a new machine all the time, but after getting a Mac Pro, I haven't bought a new desktop in over 2 years.