I think I know why so many New Year's resolutions have to do with health. There's nothing new about resolving to be healthy every January 1st. In fact, if the New Year fell in May, I think our resolved paths would vary much more wildly. Maybe it's only here in the U.S., and maybe only in the colder parts - I wouldn't know as I've lived here in the Northeast all my life (save for 5 months abroad). Heck, it may even be just Christians. But what I can tell you is, that our common resolution to be more healthy has little to do with the calender, and more to do with it's annual events and the changing weather.
Naturally, we begin October by coming indoors to socialize as the weather gets cooler. Progressively, we eat heavier foods and crave fats as the temperature drops. Apple and pumpkin season gets us ready for pies, and soon enough we're feasting our faces off at Thanksgiving. The social pull of the holidays is in full effect by the beginning of December and we get distracted from our routines by dinners, parties, planning and shopping. Parties and dinners lead to leftover food, sweets and wine which we finish either out of guilt or pure gluttony. Soon Christmas arrives with more of the same and then New Year's Eve is kind of a grand finale to the holiday season, offering us an opportunity to celebrate shamelessly.
The New Year's resolution to return to a healthy lifestyle is just the natural response to the overindulgence of the holidays. It is natural instinct to balance ourselves by resolving to act in the opposite manner as the previous months. Suddenly we're reminded very physically by our stress, weight gain and fatigue that we must get back to where we started. Other cultures or religions (for example Hindu, Muslim, and Jewish faiths) seem to be wise to this balance, and feast only after fasting and abstaining - almost as a reward. In a way, it's a cheat to pass off renewed interest in health as a resolution, when in fact we're just going around in a natural circle - even if it is backwards compared to other cultures.
What would we all come up with if we chose a different day to renew our goals? I've always been one to re-balance constantly, at the first sign of negative experience. So for now, I resolve to be more mindful of my workout routine and eating. This is only because I am feeling fat, tired, sick, and lazy, and not because it is January.