I also expect, just from my own experience, that starting a writing routine can actually cause the obsessive motivation to write, rather than vice-versa.

I started writing a musical last year while stuck in my NYC apartment during the COVID peak. At first, it was just a way to cope with the boredom. After a few weeks of writing every day, it became more and more difficult to think about anything else. Eventually I stopped, not because writing was too hard, but because it too hard to get through my day job.

I'm not implying that I have the necessary talent to succeed as a writer, but I expect successful writers feel similarly about their writing process.