Passion wanes in the absence of purpose.
This isn’t to say that every detail of every day must be done in accordance with some larger purpose. If you can, that’s great. There are things we must do simply because they must be done.
But if we operate for too long outside of our purpose, our passion will suffer.
God designed each of us to live for something larger than ourselves. Your purpose might be educating young children in a classroom. It might be feeding the hungry or working with battered women. When you discover what that purpose is, you know it. You feel it. And when you are doing tangible things towards the accomplishment of that purpose, it feeds your passion.
If you find yourself having a hard time getting motivated, that may be a warning sign to check if you are operating within your purpose.