But it's not that simple. You really have to pick and choose things that you really like and really care about. And you can't know what you like or care about until you've lived a little, which is, I think, why most young writers are terrible.
And it's hard. It's not just the self-discipline to finish things on schedule. It's the self-discipline to finish things on schedule and not have them be anything less than your best every time. Like all creative endeavors, you are constantly being judged, unfairly or not, on what you publish, and the moment you slip, it's over. It's worse, in many ways, than being a celebrity: at least they get glitzy dresses and lots of money for their troubles.
"Love what you do" is one of those clichés about work that everybody tosses around, but you can get by in most jobs without actually loving anything about what you do--you have to like it enough to do a good job, but you don't need to think about it 24/7 and unless you let it, it's not going to cut into dinner with your kids or cuddle-time with your cats. But you really have to love writing in order to make anything like a living out of it, because there's no other way to survive the constant rejections, or even the constructive criticism that your real friends* will give you.
*Seriously, take your worst POS you've ever written and show it to your friends. The good ones will tell you, "Dude, I love you, but this sucks." Those are the ones you need to keep.