I guess it's been said a few times in this thread already... but I COMPLETELY agree, that "enthusiasm" can be taught... or rather, conditioned.
Fozzie is an insanely lazy dog. If it were up to him, he would be sprawled across his favorite chase lounge day and night being fed cheetos, for the rest of his life. He also used to be a very detached, independent dog during his adolescence/puppyhood, which is a great combination for training! haha. I never thought this dog would fetch a ball. I never thought he would be into agility, or obedience, or rally, or anything compared to Gonzo.
I was so used to working with Gonzo that I didn't realize I needed to stop comparing him and treat him as an individual. Gonzo needs to be spoken to in a gentle, consistent tone... he does not require jackpots or lots of food/play reinforcement as he needs me to stay calm and let him know that he's doing good. Fozzie needs me to be excited, loud, and direct... and he needs those jackpots, lots of reinforcement and lots of repetition to keep him motivated. Using the clicker makes him 10x more into it. If I trained Gonzo in the same way, he would shut down FOR SURE. That doesn't mean training Fozzie is more difficult or time consuming, though. It's actually a lot of fun and he offers up behaviors like nobody's business. He can be the most enthusiastic dog in class, if I'm properly motivating him and if I spend the time capturing/reinforcing that enthusiasm, even if he's undoubtedly the least enthusiastic when I'm not engaging him just hanging out at home.