User blog comment:ToshiroFanGirl/Toshi's Tavern~! (Chapter 352 & Attack on Titan)/@comment-7418318-20130920120235

The Humans want to kill the Titans not over the dominion of the world. They want to kill the Titans, so the Titans stop eating Humans, be able to live outside the walls and such.

And in Erens (and many other people's case) there's also the matter of revenge for lost family members and friends.