Normal Trance is induced by a surge of emotions (or so Steiner and Kuja say).

As for Kuja (Spoilers):






His Trance isn't induced by a surge of emotion. He used the power of all the trapped souls in the Invincible to force himself to Trance. Because of this, his Trance is more powerful than anyone elses Trance (a sort of Super Trance, one might say). In short, Kuja couldn't trance the normal way, so he cheated.