User blog comment:Miskos3/Episode 272: Fairy Tail Zerø: The One Who Teaches Magic, Review/@comment-190.58.249.5-20160221003339/@comment-7418318-20160221084025

But the power of feelings isn't necessarily only the power of friendship.

There are other feelings, which going by Zeref's explanation should also give power.

Rage would be an obvious candidate.