Are they writing what they want to write, or are they only writing for the money or adulation?

A mixture. A lot of authors write in multiple genres to make ends meet (even a "major" contract pays way, way less than most people think). So for example I have one friend whose true passion is mystery with a literary bent (not a super popular niche), but also writes contemporary romance (the highest selling genre) to pay the bills.

There's also a constant conflict in publishing of writing what you love vs writing what you know will sell (not to mention what your editor will allow). So even if you pour your uncensored soul into a manuscript, your soul is going to take a beating with a red pen before it hits shelves. :)

That being said--everyone in this group writes at least one genre they're genuinely passionate about.

I've always wanted to write well, but I've never had to rely on it to make a mortgage payment, much less to establish my place in the world, and I've often wondered if that explained the fact that I don't hate writing first drafts.

I would say this is a huge factor. Deadlines have a magical ability to kill joy in a craft. I think a lot of the "I hate writing" mentality comes from "I'm a perfectionist who feels I don't have enough time to make this manuscript as good as I want it to be."

That being said--a lot of people feel this dread even before they're published, so I think there's more than one factor at play.