One of my all-time favorite (and best) managers used to describe work as pleasure with a purpose. I've always found that to be really insightful, and powerful, and for those of us lucky enough to have achieved that in our work, its very satisfying.
Steve Jobs spoke about it when he said to be satisfied you must do great work, and to do great work, you must love what you do.
Then there's the old maxim, if you love what you do, you'll never work a day in your life.
While these are nice in theory, not everyone will have the luxury of pursuing their passion (no one will ever pay to hear me sing). What's important is that you're thoughtful in your approach to choosing your positions throughout your career.
When you consider that you may only get 10-12 bites at the apple throughout your career, it's clear that every opportunity should be carefully considered. Are you going to be learning new things? Will you have fun? Do you like and respect the team? Does the company support your growth?
It's easy to 'forget' these in the heat of the moment, but if I've learned anything in my career, it's that finding the right job, the one that gives you pleasure as you're doing something purposeful, is the most important element to your mental, emotional, and professional health.