For some, it enlarges the [censored]. For others such as myself, it inspires one's completionist side.
I do like achievements and often catch myself worrying more about them than the actual game, it's just when the Xbox orginally came out and I learned about the microsoft points currency system, I thought the gamerscore earned from unlocking achievements would be used to purchase games and DLC. I was hosed.
Basically I'm just angry at them because I thought they would be worth something. Which would be nice because mine is around 25,000.