Dogs. You don't have to earn their trust; they give it unconditionally. And every time they see you is like the first and best time they've ever seen you.
The happiest sound in the world is hearing your dog smacking his tail on the floor and doing a little dance while waiting for you to unlock the door and come inside, so you can pet him and rub his belly.
The second-happiest sound in the world is the sound his feet make when he twirls in six circles trying to get comfortable before plopping down in the exact same spot he was in, giving a great big sigh, and falling asleep against your leg.