It's for the personal satisfaction. I couldn't care less about my Trophy League ranking. I'll probably never get into the top 100.

I just enjoy the game . don't care If i get trophies or not.
like I have lots of games who don't support trophies at all.

To show my superiority among the Playstation Network ! Lol I also do them for personal satisfaction.

I just got my first platinum trophy with Assassin's Creed II(haven't updated it yet)

now I'm trying do get the platinum trophy on Infamous(3 more trophies to go)

hearing that sound makes me a happier person inside, but my main reason is for competition, i hate it when my freinds are close to my trophy level, so i have to gain a lead, its fun playing a game 3 times for the plat....its totally worth it in the end

@radiantshadow02, I think that's the best part of achievements/trophies, you get more out of the game (and more for your money if you bought it). Some achievements/trophies are just stupid though (Gears of War comes to mind). It shouldn't take 3-4 weeks of serious play time (maybe 40-60 hours a week, or more?) to get all the achievements/trophies. Other than those times though, I love the system.