It depends on what you're writing about. Emotionally expressive writing has been shown to improve health, happiness, weight loss, physical healing, creativity, and reduce stress. It can help you be better at your job. Or get a new job. It will also help you to write more. All evidence to make emotionally expressive writing your only New Year Resolution.
I keep a journal. The last time I wrote in it was December 1st. Since then, I got a cold, put on weight, stressed about posting in Holidailies, and got fired.
Just kidding. I did get a cold, though, and I'm blaming it on the blank pages in my journal.
Also, writers tend to live longer than most (except for accountants).
If you really want a long and happy life, become an accountant . . . (I've never imagined myself writing that sentence. Ever.) . . and write every day.