If Wataru lost his parents when he was young, it would explain why he's so desperate to have some kind of official affirmation of his relationship with Kosame. He's afraid she will disappear as well, just like his parents did.
This is really cute but I'm a little confused, he doesn't think her kiss is romantic? They've been "together" for a decade and I guess he wants definitive proof of her love by getting that pearl... Are her words and actions not enough? Smh Wataru, smh.