Why do people act like they know the celebrities they like and ones they dislike?

I really do not know how one of my friends knows Mariah Carey is nice. She also knows that Kim Kardashian, Beyonce Knowles, and Megan Fox are conceited. Also why do people act like celebrities are incomparable looks-wise? Mariah Carey is pretty,she's not the prettiest girl ever, there are plenty of people that are prettier than her.