Passion Isn't Enough
Passion is transient.
Passion is ebbs and flows.
Passion is fickle.
Passion will get you started. It will keep you interested. But only to a point.
To keep your passion from waning, you need to put in the hard work (whether it's physical or mental). You need to keep pushing forward, challenging yourself, putting yourself into situations that are slightly out of your control.
That keeps you interested. That keeps you learning. That keeps the flame of your passion burning.