Kaeya is whoever you'd want him to be. But in his eyes? He thinks you're very special. He likes to spend so much time with you, and he definitely enjoys your presence. Even though he mostly teases you for everything.

So... why not put up a test of endurance? Maybe, true friends (or significant others) would be able to tolerate him however and whenever. Plus, he felt like annoying you.

So he took it upon himself to possibly dig under your skin. He decides to just squeak out a toot.