Besides many of the same reasons already posted, I'm a generally quiet person. It's the basic rule of "it is better to remain silent and be thought a fool than to open your mouth and remove all doubt."
Plus, a lot of people are just too argumentative. It can be exhausting to make a post and then have to defend it by posting several studies and make sure your wording can't be construed several different ways. Then, someone with a higher post count comes along and says, "That's not what I've heard" or "You're dead wrong" without anything to back it up and suddenly, all of your "defending" posts are null and void. I've been in those kinds of discussions in the past and I find it's just easier to lurk. Some people tend to not let evidence get in the way of a good "they say" theory.
I'm more likely to contribute in a thread about favorite movies, favorite songs, or favorite movie songs than a thread about "low carb vs low fat".
Finally, JohnP and I seem to have very similar beliefs and he writes a lot better than I do. I would just end up quoting his post and typing "+1". Not much a contribution.
