I just stumbled across a CBS News Report from April 2009 about the positive health advantages of knitting. This is a truth that most knitters have been aware of for years, but it now looks like scientists are starting to see the benefits. Studies have shown that the repetitive movements of knitting help with stress, pain, depression and even strengthening the immune system. The Mayo Clinic is conducted research that is leading them to believe that knitting might even "be nero-protective," helping the brain's functioning ability as we age.
So next someone asks you why you bought more yarn, you can simply tell them it's for your health and your boosting your immune system. :)