Donono was ecstatic because the lion had chosen the same difficulty as him. This way, they can dive into complete danger with no reduction in rating.
Usually, if the mount chooses lesser difficulty than the rider, then no matter how much they perform, the rating will reduce, and the rewards will be affected.
The Reason was the mount would be protected to a certain degree by the arena itself, especially from attacks that surpass its difficulty.
Now, the lion won't face any protection from the arena, and so if they win the race, their rewards will not be reduced at all.
Donono is confident in himself; he believes he can protect the lion and himself. Though he knew other powerful challengers would be there, he wasn't scared.
The more challenging the better one refined themselves.
If he died, he would only blame his weakness.