Oh the impressions I got from reading his bios is that he loved writing and being part of the science fiction world. He may have been less enthusiastic about the Foundation series later on as he perhaps felt it was stereotyping him. Kinda like how conan Doyle got tired of Sherlock Holmes and so killed him off.
Right - he did love writing but, at least in
I. Asimov (I haven't read
In Memory Yet Green/In Joy Still Felt) he especially loved non-fiction writing best of all. And, yes, being part of the SF world, but not specifically the writing of SF, as much. This is slightly out of context, but on writer's block he says "Frequently when I am at work on a science fiction novel (the hardest to do of all the different things I write) I find myself heartily sick of it and unable to write another word" and then says he doesn't let it stop him, but simply turns to writing on some other project. (p.209 of my paperback.) Talking about basically leaving SF after 1958 and expecting his income to go down, he says "However things did not work out as I expected. In the first place, writing non-fiction was much easier and much more fun than writing fiction...If I had tried to write
fiction full-time, I would have undoubtedly broken down." (p.253.) I mean, that goes without saying - virtually everyone gets writer's block or sick of writing something here and there and if he were to turn out four or five hundred books they couldn't have all been fiction - Balzac couldn't even do that. And, in other places, he gives the impression he often did enjoy writing SF and novels, but just not as much as I would have thought.
As far as the Foundation/Robot series, I gather he would have been just as happy never to start again but the way he describes getting back into it by re-reading the trilogy and the few pages of the fourth book he already had, it kind of swept him back into writing it. He sounded like he was having fun there. "Now I
wanted to write a fourth Foundation novel..." (p.466.) But even after the success of
Foundation's Edge, he reacts by saying "...I knew I was doomed. Doubleday would never let me stop writing novels again -and they never did." (p.469.) Not sure how much of that is tongue-in-cheek, but it's at least partly serious.
And then, even after realizing he "had no choice" (p.482), he didn't want to write any more Foundation stuff after that, so turned to the Baley/Olivaw books and rewrote the part of the third he had and finished it. Then he does say he enjoyed writing
The Robots of Dawn, so went on with the fourth one along with the idea of tying it into the Empire but, yeah, one gets the idea the later books were more perfunctory or Doubleday-driven.
Anyway, part of this may be that I had just read Pohl's
The Way the Future Was and, unlike Asimov, Pohl was almost purely SF, though as writer, editor, and agent. Whereas Asimov was pretty much just a writer, but a writer of much more than just SF. And Pohl goes into great detail regarding The Futurians and so on, whereas Asimov (relatively) just brushes the subject here and there.
Either way, I enjoy Asimov's non-fiction but his fiction most of all and I'm glad he wrote both.