"The way I decide which one to write is that one starts to write itself in my head."

As someone who only comments in two places on the web (HN and at another place that I'd rather not mention) that is exactly how it works when I make comments.

You read something you react you write.

It's not an assignment. It's not a job. It's not required. You just do it because you are moved and/or inspired by something you read or observe.