I remember hearing in a business meeting several years ago:
If you want to know how indispensable you are, just stick your finger in a glass of water. When you pull it out, measure the size of the hole you left behind.
It’s a fairly potent lesson in business; as an employee, don’t assume you can’t be replaced. As a business, don’t assume your client can’t replace you.
It’s easy to forget though that it works in the other direction too – if you’re worried about leaving a job out of some sense of obligation to the company or (more likely) your coworkers, you shouldn’t. If the finger-in-glass analogy is accurate, it equally means the water will do just fine without you. That’s called freedom.