But I don't go looking for songs for them, they just get added when I hear something that sounds like it belongs.

I admit, sometimes I go looking for Eros music, but I tend to scour my own collections for it, usually because I suddenly remembered a lyric fragment or I'll get an idea in my head about a song or songs with a certain word in the title, and see if it works. Usually, I was right about the song, which I chalk up to my amazing music memory, and when I'm not, I don't sweat it too much, cos I'll find something else that DOES work, sooner or later.