He'd never seen her laugh before.
In all their running and falling and getting soaked and shot at and arguing and hugging and kissing, Alice had never once laughed.
When they escaped from her mother's gimlet eyes and took to the streets of her city, Hatter eagerly told her all the amazing things he'd already learned about her world in the day he'd waited between following her through the Looking Glass and coming to her mother's apartment. And when her eyes widened and she laughed at one of his observations, he lost all track of thought.
Her eyes sparkled and her smile lit up her face, and the sound of her laughter went straight to his heart and his gut. Instantly, Hatter's lifelong ambition, which had already changed from "survive" to "Be with Alice", changed again to "make Alice laugh for the rest of our lives."