Elephant & Piggie: My Friend Is Sad by Mo Willems
Gerald is careful. Piggie is not. Piggie cannot help smiling. Gerald can. Gerald worries so that Piggie does not have to. Gerald and Piggie are best friends.
In My Friend is Sad, elephant Gerald is down in the dumps. Piggie is determined to cheer him up by dressing as a cowboy, a clown, and even a robot! But what does it take to make a sad elephant happy? The answer will make even pessimistic elephants smile.