General:
* Ardent
* Fervent
* Zealous
* Enthusiastic
* Eager
* Committed
* Devoted
* Intense
* Vehement
* Fired up
* Rapturous
*
For a specific subject or activity:
* Avid (e.g., an avid reader)
* Keen (e.g., keen on learning)
*
For love or relationships:
* Romantic
* Loving
* Affectionate
*
Stronger words:
* Furious
* Wrathful
*
The best word will depend on the specific situation and the nuance you want to convey.