I could be thinking of one of Heinlein's other books, or even just something he said randomly. I've read a lot of his stuff, and it all starts to blur together.
>"But you don't enjoy it and we don't need the money. Truly we don't!"
>"Thank you, my love. But I did not explain to you the other insidious aspect of writing. There is no way to stop. Writers go on writing long after it becomes financially unnecessary...because it hurts less to write than it does not to write."
If I come across it, I'll make a note of it.