User blog comment:Miskos3/Episode 272: Fairy Tail Zerø: The One Who Teaches Magic, Review/@comment-190.58.249.5-20160221003339/@comment-5303512-20160221123147

Remember what Hibiki said about Nirvana and the risk of Natsu turning to Darkness? He said that it would not be a likely scenario since if someone's anger is for the sake of someone they care about. Otherwise even villains would gain a significant power up just like the FT members. Power of feelings can be the feelings that connect one with others with bonds.

And in chapter 296, Mavis confirms that it is the Power feelings that gives one strength.