It seems slightly odd when you first see it, but I think it makes good sense.
Firstly, it's about attitude – it's important to enjoy your work, and you need to try to find a way to enjoy your work even if you didn't think you would. Sometimes that just isn't possible, but if you can find a way to turn any job into something you enjoy, you'll be onto a winner. If you set out to just do the kind of job that you think you'll like, you're more likely to be disappointed when you can't get it, or when you do get it and it turns out not to be what you expected.
Secondly, it's about your own motivation – there was a good article on Sitepoint recently about why turning a hobby into your main job is a bad idea, because you are likely to under-sell yourself and do work for too little payment, and you're also likely to stop enjoying it as much when it becomes a full-time job, and so you've then lost yourself a hobby as well.
I really enjoy web design, and I maintain several sites free for non-profit organisations or in another case for a small amount of advertising revenue. I've deliberately steered clear of jobs where web design plays anything more than a passing role because I know that if it became a full-time job, I would lose some of that interest and enjoyment, and probably would give up on the hobby sites as a result.