When I was much younger and was sitting around playing guitar and singing with people, I used to think “I wonder what’s the harmony for that”. Sometimes I’d bravely add a note or two.
What I have learned from composing and arranging music is that there are 10,000 harmony lines. There are some I like, some I don’t. Some I like while *others* don’t. Some that are easy to write or to sing and come so naturally it is like a stream of incense floating across the room. Others that are so challenging that it takes months to figure them out.
And it’s not just like this in composing, it’s like this in life. No two melodies alike, no two lives, and the number of ways they can go together – or don’t – are endless.