I remember 10 years ago now when I was just starting out at college and I asked my art history teacher a similar question about originality.
He said to me that originality comes naturally even if what you create seems cliche. For example we all experience love but each person loves differently. Same with hate and pain we all hate and feel pain as we're human and it's a cliche but it all is slightly different.
My teacher said to me if you just look at how many people have painted and drawn skulls in art you'd think that Damien Hirst was wasting his money making one out of diamonds.
The point is that like you said it's irrelevant. Think about all the different covers of famous songs that have become successful some even being more popular than the original. Just because the concept and idea isn't original doesn't mean the perspective or approach can't be.
Do we stop loving because we have experienced love before? No! I think a lot of artists get very bogged down by the idea of originality. They have to be the first to have thought about something or to bring something into being otherwise it's not exciting. I just don't think this is true.
RE: Why write music?