When I was younger, I used to while away time by browsing exotic vacation destinations (that I couldn't nearly afford) on travel websites. One day, I got an offer to work at one of them. After about a month there, the last damned thing in the world I wanted to spend my free time doing was looking at travel website destinations. Made me re-evaluate what sort of jobs I'd want to do day in and day out.
Now if you could figure out how to do it without either customers or bosses...
If you do what you love, what you love becomes work.
Is you do what you like, then you like your work, go home, and have energy for the things you love.
I would say that, in this context, liking something means it is satisfying to you beyond the average activity. While loving something means that it is something you consider a part of your life - something that you feel you couldn't do without.
So working at a job which involves something you love can sap the joy out of it, because you end up doing that thing not for you, but for other people.
Some programmers write code at work and then do side projects on top of that. Other people enjoy knowing a little code, but would find it burdensome to write production quality code everyday. For them, programming is a hobby. Other people are content to program for their job and then leave it at the office and stop there.
People who have a hobby they love sometimes find their enjoyment is completely ruined if they try to turn it into a business. Loving something on weekends and evenings doesn't necessarily translate to being able to do it happily all day, everyday for work when other people are calling the shots and telling you how to do it.
For programming the difference between what I like to do and what I can easily get paid to do is not that big, so it makes sense to combine as a career advice. But for hobbies like music or painting, unless you get extremely lucky, it is whole lot harder, takes much longer and requires to make much more compromises to get to the point where it is really fulfilling.
I only ever tried the latter semi-professionally. And it was already a soul crushing experience at times.
For it to make sense, at a minimum you have to also like and be pretty good at running a small business in addition to the thing that you love.
Several theories, probably something to each of them. The one I think explains the most is that if nobody is "passionate" about whatever you're doing, then you can approach things analytically, instead of having a layer of emotional investment gumming things up.
Personally, jobs that I thought the concepts/industry were interesting to me, but the work was far enough removed that it didn't sour me to the industry have worked out well. e.x. If you're a car guy, working on the software for an inspection company isn't too bad, you still come out liking cars, and wind up with some interesting stories.
OTOH, I've spoken with colleagues that have a bit more nuanced mindset: If they don't care about the product, that's fine, but they won't work for a company that sells a product they know they don't like, they won't do it.