User blog comment:SageSlayer/Why does Erza treat Natsu differently than others?/@comment-26184007-20170228020624/@comment-31119762-20170228021438

I also figured that as well. Natsu did save Erza's life in the Tower of Heaven arc as well. It seems she is a little biased towards him though. Also to Wendy as well but moreso Natsu.