I never really "wanted" one, just so happens I was blessed with a little more junk in my trunk than others.. And honestley its been nice having a big booty, guys have gone crazy over my a$$ ever since I can remember. And now that I'm married, turns out my hubby is a "butt guy" and compliments and obsesses over my bodacious behind daily.. Its nice to have a feature that's praised so much by the opposite sex. So I guess that's why women want their own butts to be big, because its attractive to men... But most caucasion females I know want the exact opposite, they usually want to be toothpicks with no jiggle in their jeans.
I like having one (well I think it's more on the medium side but w/e) because it looks better in jeans/pants for the most part, I like how I look when I wear underwear, and to be honest I think it's overall better to have a pear shape (hips, thighs, butt) than be top-heavy.