"He was a lot of things to me" "The only conclusion was love"
I think it’s a combination of nature and nature. PP said that it was not a coincidence that Henry is the product of dark and light magic. So, Henry’s lineage seems to be really important about why Henry’s heart is special, but I think that Henry had to believe in the dark curse for him to become the truest believer.
Not only believe in the dark curse, but believe that it could be broken. My favorite Henry moment is still right before he eats the turnover of S1 and tells Emma that he believes in her. It’s one thing to believe in the curse, it’s another to believe that it could be defeated. If Henry hadn’t believed so poewrfully in Emma then it never would have been broken.