As a general rule, it's a good idea to let people eat what they want and leave them alone about it. If someone wants to be vegan, who cares? Let them be vegan. If someone is avoiding gluten, fine! So long as they're not trying to take the donut out of your mouth (which is an easy way to lose a finger) or make it harder for people with real allergies to be taken seriously, then let them avoid gluten. All the more donuts for you! An adult's personal diet isn't really anyone else's business, so it's weird to harass someone for not making the same dietary choices that you make.
Unless we're talking about the paleo diet, which we can all agree is a dumb diet for dumb people who all need to be told how dumb they are.
Joking, of course. If you're a man who wears toe shoes and keeps his hair in one of those greasy half-ponytails, then the paleo diet might be perfect for you. In fact, as mockable as the idea of eating like a hominin of the paleolithic age is, the paleo diet does have some health benefits. Cutting out processed foods (the paleo diet forbids any consumption of dairy, processed grain or processed sugar because they weren't in use pre 10,000 BP) and eating more proteins and vegetables can lead to a more nutrient rich diet.
That said, anyone who thinks that they're actually adhering to a legit paleo diet is kidding themselves. As Ferris Jabr points out in the Scientific American, both our bodies and the foods we eat have evolved greatly in the last 2.6 million years, so what you think is a caveman friendly meal of meat and tomatoes is actually an entirely different meal from what the first humans ate.
As Christina Warinner of the University of Zurich emphasizes in her 2012 TED talk, just about every single species commonly consumed today—whether a fruit, vegetable or animal—is drastically different from its Paleolithic predecessor. In most cases, we have transformed the species we eat through artificial selection: we have bred cows, chickens and goats to provide as much meat, milk and eggs as possible and have sown seeds only from plants with the most desirable traits—with the biggest fruits, plumpest kernels, sweetest flesh and fewest natural toxins.
Jabr also points out how our own bodies have evolved to support lactose and processed foods over time. Again, if you feel better when not eating dairy, processed sugar or processed grains, then by all means forgo those things, but don't cloak your dietary choices in a wishy-washy ideology about the health of the first man. It's downright impossible to replicate that diet and, as it turns out, the people of the paleolithic era weren't all that healthy to begin with. A study of over 100 mummies from societies of farmers, foragers and hunter-gatherers around the world found that 47 of the 137 bodies had hearts that showed signs of clogged arteries (atherosclerosis).
Anyway, enjoy whatever diet is best for you. That's your prerogative, but if you're gonna go paleo keep in mind that what you're eating is not really paleo friendly. Still, that doesn't mean you have to enjoy your animal fats or wilderness parkour any less.
Paleo Diet Is Half Baked [Scientific American]