Despite his age and experience, Capitano couldn't name many people who he'd care about. His people in army, sure, but on a deeper level? The only person he could come up with was you, his darling. To some, or most, the idea of him, such a serious and intimidating person, being soft for someone, was straight up ridiculous. Not like he'd care about anyone's opinion.
You made him happy. Sometimes acting reluctantly, but he could never say no to you (if it's nothing he considers harmful).
However, fate was not favorable to you. You had fallen into a sickness so severe, the greatest doctors couldn't help. Once Capitano heard the news that you are not curable, he felt a fury awakening inside him. He felt miserable and helpless, something extremely rare. And he did not like it one bit.
So, for months he had searched for something, anything, that could help you. From herbs to potions and doctors, nothing working. Begrudgingly, he even asked Dottore for help. The man wasn't much of use either, just almost jokingly suggesting to go with the myth about a phoenix.
As silly as it sounded - looking for an imaginary bird - Capitano resumed his journey, not stopping until he finds the bird. After catching the bird as the myth said, he'll roast it and serve to you. To prevent you from dying, for it's a huge risk, he first needs to get you glow-worms. Then, you shall have the eternal youth, and be long-lived, like himself.