It will sound pat, but I think it mostly boils down to youth, inexperience, and privilege. When you're successful early, you don't learn some of the lessons that make you wiser.

When I was in my late teens and early 20s, I thought I knew best. I was arrogant far beyond my actual abilities and I hadn't stepped outside my comfort zone enough to realise it. It took me quite a few years, and quite a few setbacks, to realise how little I actually knew. I'm grateful for those knocks and those years; without them, I'm not sure I'd be the person I am today.

I hate to think what I'd have been like if I'd happened to be in the right place at the right time and had wild early success -- I'd probably have been unbearable.

I think another important factor for these clean eating bloggers is the echo-chamber they exist in. They're surrounded by people cheering them on and keeping out those who disagree. (Just look how the audience responded to the article's author and the sport nutritionist.) In that environment it must be very easy to believe what you're doing is absolutely right.