By starting out with riffs and fills, you subconsciously develop playing patterns, just as you do with strumming patterns. As a consequence, certain things might be easier for you to play than others. I started out doing lead patterns based on chords and branched out from there. This is where things can get a bit confusing. The song is in E minor.
E minor is the relative minor of G major.
And this is indeed the case. In most cases, the music will initially do that for us. For the record, when I teach this song to my students I don't even dignify it with a name. It really and truly saves a lot of time that be spent enjoying the music itself. This would be much in keeping the center of the song in E minor and that is something that is not open to debate.