Welcome home. This is the place where you belong. The place to which you shall offer yourself... is here. 

Unlike the rest of them, Garland gave me a soul, that I may replace you and Kuja. But Garland is dead. Terra will soon be destroyed. What's the point?

Planets have a cycle of souls. Souls are born from the planet, and then return to it. Garland planned to gain control of that very cycle... and you were supposed to help him achieve that.