He is addicted to the sound of her voice. It's a symphony of half understood babble and it's as easy to get a headache from as a marching band, yet he'd rather listen to nothing else. She is his drug. She is all he needs. She is perfect. He wants to be there for her always and to be able to spend the rest of eternity protecting her.
She is addicted to the gravelly tone of his voice. It's hypnotic. Mesmerizing. It makes her feel safe. He is able to soothe her nerves with a smile. He is hers. Forever.