Matt Yglesias tries to make the case for a soda tax:
Taxing the work people do can have a net beneficial impact on the economy if the tax revenue is spent on something adequately useful. But all else being equal, it does create a drag on the economy. Taxing cigarettes and soda and so forth, by contrast, mostly pushes people toward better healthy outcomes and therefore does something to boost quality of life and economic growth.There's a difference between cigarettes and soda. If you smoke cigarettes, I have to breath in your second-hand smoke. So taxing cigarettes makes sense. Your action makes me less healthy, so you should be taxed for doing it. On the other hand, you drinking soda doesn't affect me in the least. It does make you less healthy, but maybe you'd rather drink soda than be healthy. Why should the government decide that?
(Okay, you could argue that since I pay for Medicare/Medicaid/etc. and soda makes you less healthy, eventually I'll have to pay for your bad decisions, so it does affect me. But this justifies taxing all kinds of things, like skydiving and not taking a higher-paying job.)