There is one point that was missed in this post: promotion. Are you more likely to land a promotion in your current job, or are you more likely to find a another job that is also a promotion to a position that you have no experience in?
Four years down the line, do you want companies looking at your CV and seeing you change jobs every 10-12 months? How good an investment are you as a hire if you're going bail shortly after you're up to speed and a productive member of the team?
I would be wary about someone who kept leaving after 12 months and if interviewing I would ask them to explain why. On the other hand if you have quantifiable things that you have done/built during your tenure there shouldn't necessarily be an issue. It could be a simple reason like your partner's job kept changing location and you moved with them.
Not to mention that if you have a family, kids, a mortgage (i.e. a stable life), moving jobs every year isn't in any way practical.