If there was one thing Clara loved, it was silliness. It was a chance to escape your world, for a second, for a singular moment, with the smile on someone's face, or the made up world that you could pretend was real. The happiness on a child's face always made Clara smile. It was why she loved her job. With made up stories of fishes and clock towers Clara Oswin Oswald could put a smile on a child's face. By introducing an entirely different reality, she could join the child in it. Even for a second, believing she had an entirely different life than the governess and bartender in London. Then, she met the Doctor. He was the silliest man she'd ever met. But, silliness is just a mask of the sadness, and the pain deep inside him. Clara knew that he was hurting, but they were actors, constantly masking the pain with laughter. This is why she liked him. He was so silly, but it was just a masking of all of the pain inside him. And Clara saw right through that damn mask.