Here's my guess: You hate it because you're probably one of them, no? Everyone hates a label, because it's an attempt to put your beliefs, aspirations, opinions in a box. And, therefore, to tell you you're not a person so much as a product of your environment. Of course we all suffer from circumstance, and we share a lot with those who grew up at a similar time, culturally speaking. But we still each have an identity, and letting others tell us what our identity is seems like a robbing of our fundamental right of self-determination. Millenials in particular have suffered from coming of age in a once-in-three-generations terrible economy, so unemployment among them is higher than what one would expect on average for the last hundred years. Old people see this and ascribe it to laziness, and not circumstance (ironically). Therefore, "millenial" has become a pejorative euphemism that essentially means hopelessly unemployed, lazy complainer. Just my conjecture.