At best, vegans are doing what they can to protect animals, the environment, and their health. At worst, vegans are judgmental preachers trying to ruin your 3 AM fast food snack.
While there are plenty of wonderful things about a vegan diet, it's never a good idea to attack someone else's dietary choices. Most of the time, regardless of how convincing your argument is, you can't break someone else's habits.
Vegan: I'm a vegan.
Meat-eater: That's not healthy. How do you get your protein?
Meat-eater: Look at our canine teeth!
Vegan: *walks away*