vegan vs vegetarian

Ms
I see a LOT of people against veganism(saying it's unnecessary,not healthy, things like that). Why so much hate for the vegan lifestyle, and so you feel the same way about vegetarianism?
The only major difference is that vegetarians can have dairy(and honey and eggs)...but mostly dairy.
Since diary isn't healthy for humans anyways, why is veganism so "wrong" to some bit being vegetarian isn't just as "wrong"?