User blog comment:Stone Roger/Natsu and Zeref have somekind of relation?/@comment-50.83.59.209-20140208032250

I think that Zeref is Natsu's father first because he said "you've grown" or something like that. and second maybe Zeref got lonely over those 400 years and met Natsu's mother. Natsu was born but, Zeref couldn't stay and Natsu's mother some how couldn't take care of him so Igneel found him and took him in. But, that is completely a guess so don't take it seriously.