User blog comment:Firegod00/In Which Fire Quits (FT 433 &434)/@comment-26307239-20150511221141

A hypothesis I always believed in, though it has its problems in it. Taking My Little Pony's quote, Friendship is Magic. That means Friendship is also Magic. The more Friendship you have, the more Magic you have. Since Natsu is going about his values in friends and comrades, those convert into magical energy which gives him a boost of magical power so he can save the day like always.

(This is my logic, don't judge.)