Let me try to explain it.

Think of your character and how he would act. If he is a fire giant, he may think of a few things (and a few things alone) in his life. War, Hate, Lust, Greed etc. Now pick the most important thing to him, the emotion/drive that he thinks of first and foremost before making actions. That would be his sphere.

If I am a paladin who would die before bowing to an enemy, I may be sphere pride (because I'm too proud to). However, if that same Paladin won't bow because he hates the darkness rather then just feels prideful, he may be sphere Destruction (destruction of the evil in Thera).

Hope that explained it better